Bump GTK requirement

It requires for close button and for
fallback app menu in a headerbar.

https://bugzilla.gnome.org/show_bug.cgi?id=705909
Add headerbar and set that as titlebar

https://bugzilla.gnome.org/show_bug.cgi?id=705909
Fallback app menu: set app icon

https://bugzilla.gnome.org/show_bug.cgi?id=705909
Set headerbar title based on calculator mode

https://bugzilla.gnome.org/show_bug.cgi?id=705909
Depend on unreleased Vala 0.23

Needed for the GTK+ 3.11.2 bindings.

https://bugzilla.gnome.org/show_bug.cgi?id=705909