Avoid leaking a GtkMenu

This is only freed on application exit, but in as far as there's
a point to freeing it at all we should do it properly.
Our handling of this GtkMenu is "unique" - it isn't owned by
anything, and we paint it ourself manually. Therefore, in order to
handle the refcounting correctly, we should sink the initially
floating reference, and explicitly unreference it to finalise.
Valgrind confirms that the leak is eliminated by doing this.

Change-Id: I4c796c4d37dd8f514aff9768fab66168d36bae64
Reviewed-on: https://gerrit.libreoffice.org/11682
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>