topbar: Gd.MainToolbar -> Gtk.HeaderBar

Lets try to reduce oure dependency on Gd as much as possible and in this
particular case we need to do this to allow for client-side decoration
that will be introduced in a following patch.

https://bugzilla.gnome.org/show_bug.cgi?id=706414
machine: Remove a redundant call

This call should have been deleted as part of commit
33883def2a47fb477d2f0fb4928c8777466f643b (Fix positioning of thumbnail
in properties view).

https://bugzilla.gnome.org/show_bug.cgi?id=706414
topbar,app: Switch to client-side decoration

In preparation for the wayland tech demo in 3.10, all other GNOME 3 apps
are switching to client-side window decoration so we should too.

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