UI|Client: Animate widget opacity when enabling/disabling them

It is good to avoid abrupt visual changes.
UI|ToggleWidget: Ensure a disabled toggle’s state can be seen

The “flipper” of the toggle is now drawn with a larger-than-one
alpha so that it will be opaque even if the widget as a whole has
transparency.
UI|Client: Keep task bar hidden until app startup is complete

Looks cleaner this way.