Use avatar chooser headerbar only when requested

Use headerbar in the avatar chooser dialog only when necessary according
to the "gtk-shell-shows-menubar" setting.

https://bugzilla.gnome.org/show_bug.cgi?id=728866
Use a traditional title bar when requested

Not all environments desire header bars. We use the
'gtk-shell-shows-menubar' setting to decide whether to use the header
bar.

https://bugzilla.gnome.org/show_bug.cgi?id=728866