GtkBuilder: change format of menus

GtkBuilder: change format of menus

Change the format of GtkBuilder <menu> to be more in-line with the style
of the rest of GtkBuilder so that we can do translation in a consistent
way.

The format is now substantially more difficult to hand-write, but tools
should be along soon.

There is an xslt program attached to the bug to help you convert your
existing .ui files from the old format to the new one.

https://bugzilla.gnome.org/show_bug.cgi?id=668696