require at least gtk3 3.20.0 to build and run gtk3 bits

Change-Id: I2caf8c32d47ce195a3b714e50256d8425a5a117e
Reviewed-on: https://gerrit.libreoffice.org/45735Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/inc/unx/gtk/gtkgdi.hxx

@@ -131,9 +131,7 @@ public:
 
     virtual void GetResolution(sal_Int32& rDPIX, sal_Int32& rDPIY) override;
 
-    GtkStyleContext* createStyleContext(gtk_widget_path_iter_set_object_nameFunc set_object_name, GtkControlPart ePart);
-    GtkStyleContext* createNewContext(GtkControlPart ePart, gtk_widget_path_iter_set_object_nameFunc set_object_name);
-    GtkStyleContext* createOldContext(GtkControlPart ePart);
+    GtkStyleContext* createStyleContext(GtkControlPart ePart);
     GtkStyleContext* makeContext(GtkWidgetPath *pPath, GtkStyleContext *pParent);
 private:
     GtkWidget       *mpWindow;

vcl/unx/gtk/gtkinst.cxx

@@ -82,7 +82,7 @@ extern "C"
             XInitThreads();
 
 #if GTK_CHECK_VERSION(3,0,0)
-        if (gtk_minor_version < 18)
+        if (gtk_minor_version < 20)
         {
             g_warning("require a newer gtk than 3.%d for theme expectations", gtk_minor_version);
             return nullptr;

vcl/unx/gtk/gtksalmenu.cxx

@@ -630,12 +630,7 @@ void GtkSalMenu::ShowCloseButton(bool bShow)
       "min-width: 18px;"
       "min-height: 18px;"
       "}";
-    const gchar olddata[] = "* { "
-      "padding: 0;"
-      "margin-left: 8px;"
-      "margin-right: 8px;"
-      "}";
-    gtk_css_provider_load_from_data(pProvider, gtk_check_version(3, 20, 0) == nullptr ? data : olddata, -1, nullptr);
+    gtk_css_provider_load_from_data(pProvider, data, -1, nullptr);
     gtk_style_context_add_provider(pButtonContext,
                                    GTK_STYLE_PROVIDER(pProvider),
                                    GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);

vcl/unx/gtk3/gtk3gtkframe.cxx

@@ -2081,9 +2081,6 @@ void GtkSalFrame::grabPointer( bool bGrab, bool bOwnerEvents )
     if (!m_pWindow)
         return;
 
-#if GTK_CHECK_VERSION(3, 20, 0)
-    if (gtk_check_version(3, 20, 0) == nullptr)
-    {
     GdkSeat* pSeat = gdk_display_get_default_seat(getGdkDisplay());
     if (bGrab)
     {
@@ -2094,24 +2091,6 @@ void GtkSalFrame::grabPointer( bool bGrab, bool bOwnerEvents )
     {
         gdk_seat_ungrab(pSeat);
     }
-        return;
-    }
-#endif
-
-    //else older gtk3
-    const int nMask = (GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK | GDK_POINTER_MOTION_MASK | GDK_POINTER_MOTION_HINT_MASK);
-
-    GdkDeviceManager* pDeviceManager = gdk_display_get_device_manager(getGdkDisplay());
-    GdkDevice* pPointer = gdk_device_manager_get_client_pointer(pDeviceManager);
-    if (bGrab)
-    {
-        gdk_device_grab(pPointer, widget_get_window(getMouseEventWidget()), GDK_OWNERSHIP_NONE,
-                        bOwnerEvents, (GdkEventMask) nMask, m_pCurrentCursor, gtk_get_current_event_time());
-    }
-    else
-    {
-        gdk_device_ungrab(pPointer, gtk_get_current_event_time());
-    }
 }
 
 void GtkSalFrame::CaptureMouse( bool bCapture )