GtkMenuTracker: Don't return FALSE as a pointer

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741249
GtkPlacesSidebar: Don't return FALSE as a pointer

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741250
GtkWidget: Don't return FALSE as a pointer

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741251