[applets/kicker] Explicitly set visible property when opacity is 0

The Dashboard representation contains 3 views stacked on top of each
other with one visible at a time with an opacity animation.

Enabled was used (presumably) to ensure that only the active
representation gets input events.

Unfortunately this has a side effect. When switching views the icons on
the inactive view all get enabledChanged events. This triggers a redraw
with a greyed out effect on all icons. This is a very slow path,
especially for something not even visible!

Setting visible also stops input events, without the side effect.