New style menu

Matthias Clasen requested to merge new-style-menu into master

Assorted changes to GtkModelButton and the popover generation machinery.

GtkPopoverMenu has been updated to be more menu like.

A new GtkPopoverBar widget has been added that can play the role of menubar with popovers.

Some initial Adwaita styling is included.

Edited by Matthias Clasen

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