Enable popovers in menus.

Modified the glide file to use a ``<menu>`` rather than the old ``<GtkMenu>``.
This results in an abstract MenuModel instead of a specific GtkMenu. The model can then be used by popovers. So this enables popovers while hopefully maintaining compatibility to GTK 3.10.
But this also means that all icons in the two menus will be gone. I hope you are willing to sacrifice the icons. GTK 3.10 deprecated them.
Check it out and tell me what you think.

Cheers
Jay