Fix build

Oops, this was cherry-picked from gnome-3-10 and not all of these
signals still exist.

Turns out the signal misfires are a Vala codegen mistake when it creates
a fundamental type, and can be avoided by making game a GObject.
Always enable New Game on game over

If something goes wrong with the engine before the first move, then the
user needs to be able to press New Game.