Fix #1

[chaorace]

This change makes it so that we internally track the timer state as a new Animation property called timer_running. Internal tracking is used instead of observing a property of self.timer, because the "timer" in this case is actually just an ID.

The type check against self.timer has also been removed, because self.timer will always either be of type nil or string, since it is exclusively populated by the return value of glib.timeout_add.

These two changes make it possible to properly emit the anim::animation_stoped signal and clean up the glib source when animations are forcibly stopped via stopAnimation (see #1)