expander-box: use a separate ClutterAlpha for each timeline
expander-box: use a separate ClutterAlpha for each timeline

Using two separate ClutterAlphas for open_timeline and expand_timeline
avoids the need to swap the timeline in a single ClutterAlpha during paint
and allocate, therefore avoiding the cost of the notify signal emission
associated with calling clutter_alpha_set_timeline.

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer