Don't implement popup_menu in GtkWindow

This leads to disastruous results, since each menu is itself
in a GtkWindow, so holding down the menu key leads to a neverending
cascade of menus on top of menus.

https://bugzilla.gnome.org/show_bug.cgi?id=722106
Merge pull request #7 from mtwebster/popup-menu

Don't implement popup_menu in GtkWindow