So creating menus from gtkbuilder and attaching them to a button
makes them inaccesible to be freed afterwards.
I expected setting the button model to NULL to freed them, but it's
not happening, and trying to unrefing the menu from outside gtkbuilder
only free the main menu, but not some children (which I guess they are 
ownership by the menu button and therefore not freed).
Attached are one patch for gmenu to count the number of created menus
and finalized menus.
The ohter patch is a hacked testpopover just to show the menu leak.