Depend on GLib 2.40

For g_application_handle_local_options()
Start up GApplication-style

* Create the window in activate(), not startup()
* Override the handle_local_options() signal
* Override the open() signal
Remove references to a "2D/3D chess game"

Might not be 3D for much longer, and 3D is no longer an awesome feature
so there's no point in advertising it.