Add first attempt at an error message etc API

Add a touch_ui_dialog_modal() function which in some way displays an
error or other message to the user, and possibly offers the
possibility to choose some "OK", "Cancel", "Retry", "Ignore" type of
response. Note that the "modality" is from the point of view of the LO
thread.

Change-Id: I1b2ad15854f8d76e2c30f93fb5356fad7e5d02ca

include/touch/touch.h

@@ -57,6 +57,27 @@ void touch_ui_show_keyboard();
 void touch_ui_hide_keyboard();
 bool touch_ui_keyboard_visible();
 
+// Dialogs, work in progress, no clear plan yet what to do
+
+typedef enum {
+    MLODialogMessage,
+    MLODialogInformation,
+    MLODialogWarning,
+    MLODialogError,
+    MLODialogQuery
+} MLODialogKind;
+
+typedef enum {
+    MLODialogOK,
+    MLODialogCancel,
+    MLODialogNo,
+    MLODialogYes,
+    MLODialogRetry,
+    MLODialogIgnore,
+} MLODialogResult;
+
+MLODialogResult touch_ui_dialog_modal(MLODialogKind kind, const char *message);
+
 typedef enum {
     MLOSelectionNone,
     MLOSelectionText,