Bump requirements for GTK+

For headerbar in dialogs and for popover.

https://bugzilla.gnome.org/show_bug.cgi?id=727068
Use headerbar in the commit dialog

https://bugzilla.gnome.org/show_bug.cgi?id=727068
Use headerbar in the clone dialog

https://bugzilla.gnome.org/show_bug.cgi?id=727068
Use popver for the gear menu

https://bugzilla.gnome.org/show_bug.cgi?id=727068
Use headerbar in the author details dialog

https://bugzilla.gnome.org/show_bug.cgi?id=727068
Use headerbar in the preferences dialog

https://bugzilla.gnome.org/show_bug.cgi?id=727068