PC3/BusyIndicator: Revert RotationAnimator back to property interceptor syntax

Saves on an extra unneeded ID, and adds docs explaining why the `running`
property can't be bound, and a procedural function is needed at all.

This is also a slight optimization, as property interceptors are
directly QObject-parented to their surrounding context, and don't go
to a "default property" list like `children` or `data`.

Test plan: verify that starting an app with a non-running BusyIndicator
does not consume any amount of CPU.