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
Add some GtkClipboard annotations

Add annotations to gtk_clipboard_set_with_data() and
gtk_clipboard_set_with_owner(). Its not clear why these were
skipped earlier.