pieTimer: Use custom GObject property for animating pie

pieTimer: Use custom GObject property for animating pie

Before the move to Clutters implicit animations with 0846238f6 and
bf497ed64, we used Tweener to do a fake-animation of the opacity and
Tweeners `onUpdate` signal to queue a repaint of the PieTimer everytime
Tweener tries to update the animation.

Now, with Clutters implicit animations, there is no `onUpdate` signal
anymore and also `notify::opacity` no longer gets emitted since the
value doesn't actually change. This lead to the PieTimer no longer being
repainted, which broke the animation.

Fix this by implementing the current angle of the pie using a custom
GObject property `angle` and animating this property using the new
`actor.ease_property` method.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1533