topbar: Gd.MainToolbar -> Gtk.HeaderBar
topbar: Gd.MainToolbar -> Gtk.HeaderBar

Lets try to reduce oure dependency on Gd as much as possible.

This also gets rid of the workaround for libgd bug#698289.

https://bugzilla.gnome.org/show_bug.cgi?id=706414

vbenes/gnome-boxes,vbenes/gnome-boxes