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.

Switch to a start/stop API instead. Also, update to the first frame at
startup while we are at it, since this is the expected behavior.

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