app, libgimpwidgets: new gimp_event_triggers_context_menu() and use it…

… for the container tree view contextual menu.

A very annoying point of contextual menus is that they happen on button
press whereas menu item selection happens on button release. When the
menu corner is positionned on the click position, nothing bad happens;
yet when place is missing on screen, the menu might get positionned over
the pointer position. And worse, the mouse position might be just over
an activatable menu item. So we end up in this weird situation where a
click implies: press, menu opens, release, random item (whatever is
below the pointer) is selected and menu closes.

To get rid of this weird case, let's have our contextual menu happen on
button release. In reality, I don't think anyone cares that it happens
on press or release, you just "click". But what you certainly don't want
is to click random menu items!

libgimpwidgets/gimpwidgetsutils.h

@@ -30,6 +30,9 @@ G_BEGIN_DECLS
 /* For information look into the C source or the html documentation */
 
 
+gboolean             gimp_event_triggers_context_menu (const GdkEvent   *event,
+                                                       gboolean          on_release);
+
 GtkWidget          * gimp_grid_attach_aligned        (GtkGrid           *grid,
                                                       gint               left,
                                                       gint               top,