Use GTK HeaderBar, kill the menubar, remove duplicates and relocate remaining items
@jeff
Submitted by Jeff F.T. Assigned to Jeff F.T. @jeff
Description
GtkHeaderBar is friggin awesome: it allows saving a lot more space, like GtkWindow's set_hide_titlebar_when_maximized method, except that this time it allows keeping the window manager's close button on the same line as other things you can put there... such as our toolbar items.
Meaning that we could kill the menubar, the window decorations, and merge the primary toolbar into the headerbar (whether this "looks" right is an open question however).
This would save so much space that fullscreen mode arguably wouldn't even make sense anymore and we could kill that code and make our lives simpler.
This depends on bug #T3016, which in turn depends on killing application.py in favour of GTK Application.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=708375