dedicated gnome-ssk-askpass3 source

dedicated gnome-ssk-askpass3 source

Compatibility with Wayland requires that we use the gdk_seat_grab()
API for grabbing mouse/keyboard, however these API don't exist in
Gtk+2.

This branches gnome-ssk-askpass2.c => gnome-ssk-askpass3.c and
makes the changes to use the gdk_seat_grab() instead of grabbing
mouse/focus separately via GDK.

In the future, we can also use the branched file to avoid some
API that has been soft-deprecated in GTK+3, e.g. gtk_widget_modify_fg