Add default shortcuts for Find Usage, Forward/Back, Make Object

Note: This doesn't affect existing user shortcuts.

Ctrl-Shift-D    Find Document Usage
Ctrl-Shift-E    Find [Session] Usage (one letter after doc usage and
                close on the keyboard)

These are standard shortcuts for browsers:
Alt-Right   Forward
Alt-Left    Back

This matches the build/make symmetry for compile (F8):
Shift-F8    Make Object

doc/geany.txt

@@ -3428,12 +3428,12 @@ Next message                                              Jumps to the line with
 Previous message                                          Jumps to the line with the previous message
                                                           in the Messages window.
 
-Find Usage                                                Finds all occurrences of the current word (near
+Find Usage                      Ctrl-Shift-E              Finds all occurrences of the current word (near
                                                           the keyboard cursor) or selection in all open
                                                           documents and displays them in the messages
                                                           window.
 
-Find Document Usage                                       Finds all occurrences of the current word (near
+Find Document Usage             Ctrl-Shift-D              Finds all occurrences of the current word (near
                                                           the keyboard cursor) or selection in the current
                                                           document and displays them in the messages
                                                           window.
@@ -3450,11 +3450,11 @@ Go to keybindings
 =============================== ========================= ==================================================
 Action                          Default shortcut          Description
 =============================== ========================= ==================================================
-Navigate forward a location                               Switches to the next location in the navigation
+Navigate forward a location     Alt-Right  (C)            Switches to the next location in the navigation
                                                           history. See the section called `Code Navigation
                                                           History`_.
 
-Navigate back a location                                  Switches to the previous location in the
+Navigate back a location        Alt-Left  (C)             Switches to the previous location in the
                                                           navigation history. See the section called
                                                           `Code navigation history`_.
 
@@ -3636,7 +3636,7 @@ Make all                        Shift-F9                  Builds the current fil
 Make custom target              Ctrl-Shift-F9             Builds the current file with the Make tool and a
                                                           given target.
 
-Make object                                               Compiles the current file with the Make tool.
+Make object                     Shift-F8                  Compiles the current file with the Make tool.
 
 Next error                                                Jumps to the line with the next error from the
                                                           last build process.

src/keybindings.c

@@ -432,18 +432,20 @@ static void init_default_kb(void)
 	keybindings_set_item(group, GEANY_KEYS_SEARCH_PREVIOUSMESSAGE, NULL,
 		0, 0, "menu_previousmessage", _("Previous Message"), LW(previous_message1));
 	keybindings_set_item(group, GEANY_KEYS_SEARCH_FINDUSAGE, NULL,
-		0, 0, "popup_findusage", _("Find Usage"), LW(find_usage1));
+		GDK_e, GDK_CONTROL_MASK | GDK_SHIFT_MASK, "popup_findusage",
+		_("Find Usage"), LW(find_usage1));
 	keybindings_set_item(group, GEANY_KEYS_SEARCH_FINDDOCUMENTUSAGE, NULL,
-		0, 0, "popup_finddocumentusage", _("Find Document Usage"), LW(find_document_usage1));
+		GDK_d, GDK_CONTROL_MASK | GDK_SHIFT_MASK, "popup_finddocumentusage",
+		_("Find Document Usage"), LW(find_document_usage1));
 	keybindings_set_item(group, GEANY_KEYS_SEARCH_MARKALL, NULL,
 		GDK_m, GDK_CONTROL_MASK | GDK_SHIFT_MASK, "find_markall", _("_Mark All"), LW(mark_all1));
 
 	group = keybindings_get_core_group(GEANY_KEY_GROUP_GOTO);
 
 	keybindings_set_item(group, GEANY_KEYS_GOTO_BACK, NULL,
-		0, 0, "nav_back", _("Navigate back a location"), NULL);
+		GDK_Left, GDK_MOD1_MASK, "nav_back", _("Navigate back a location"), NULL);
 	keybindings_set_item(group, GEANY_KEYS_GOTO_FORWARD, NULL,
-		0, 0, "nav_forward", _("Navigate forward a location"), NULL);
+		GDK_Right, GDK_MOD1_MASK, "nav_forward", _("Navigate forward a location"), NULL);
 	keybindings_set_item(group, GEANY_KEYS_GOTO_LINE, NULL,
 		GDK_l, GDK_CONTROL_MASK, "menu_gotoline", _("Go to Line"), LW(go_to_line1));
 	keybindings_set_item(group, GEANY_KEYS_GOTO_MATCHINGBRACE, NULL,
@@ -569,7 +571,7 @@ static void init_default_kb(void)
 		GDK_F9, GDK_SHIFT_MASK | GDK_CONTROL_MASK, "build_makeowntarget",
 		_("Make custom target"), NULL);
 	keybindings_set_item(group, GEANY_KEYS_BUILD_MAKEOBJECT, NULL,
-		0, 0, "build_makeobject", _("Make object"), NULL);
+		GDK_F8, GDK_SHIFT_MASK, "build_makeobject", _("Make object"), NULL);
 	keybindings_set_item(group, GEANY_KEYS_BUILD_NEXTERROR, NULL,
 		0, 0, "build_nexterror", _("Next error"), NULL);
 	keybindings_set_item(group, GEANY_KEYS_BUILD_PREVIOUSERROR, NULL,