panel: programmatic anim. control of AnimatedIcon

panel: programmatic anim. control of AnimatedIcon

The AnimatedIcon does not have an API for controlling the animation but
relies on the :visible property changes to start and stop a timeout used
to update the frame.

This has the inconvenient of having a side effect when visible is set to
true multiple times, and is not really the API expected from such
component. Also, there is a race if it is displayed before the images
are loaded: there is no child yet and thus we get this._frame = NaN
which leads to a crash.

Switch to a play/stop API instead, and add a load event callback to the
TextureCache.load_slice_image to exactly know when we can start using
the images.

https://bugzilla.gnome.org/show_bug.cgi?id=687583