Fix some Gtk::Menu memory leaks

Fix some Gtk::Menu memory leaks

A Gtk::manage()d widget will be deleted when its parent container
is destroyed. Top-level context menus are not inside a container and
hence need to be manually deallocated.

The solution here is to use a shared Gtk::Menu pointer that is
centrally de/re-allocated.

This works because the GUI is single-threaded and at most one
context menu is visible at a time.