special case runtime path lookups to bundling builds.

* gladeui/glade-popup.[ch]: Added glade_popup_is_popup_event () to check the correct
GdkEventButton state for a context menu (in a platform independant way).

* gladeui/glade-base-editor.c, gladeui/glade-editor-property.c, gladeui/glade-palette-item.c,
gladeui/glade-placeholder.c, gladeui/glade-widget.c, gladeui/glade-inspector.c: Use new
popup function to detect whether to show a context menu or not (bug 587128).

* gladeui/glade-base-editor.c: Fixed crashes with editable treeview column types (bug 586715).