PC3/BusyIndicator: Fix hang & rapid jump on start

Not only this patch simplified and optimizes binding of animationRunning,
but it also actually fixes the bug when an indicator would fade in still
and then rapidly rotates to catch up with its animation state.

Extra ID and binding on opacityAnimator.running is required due to the
way Animator types work (they only trigger setter and property change
signal when they stop).