Use Show desktop item instead of empty text in TabBox

If there are no windows for Alt+Tab the Show Desktop entry
is shown. This works better with both layouts and effects.
Before this change effects were not activated at all but
a fallback to the layouts was used. Concerning the layouts
some looked rather bad. E.g. Thumbnails did not show a
thumbnail but a legacy text had been shown.

When a window gets added to the empty list, the show desktop
window is removed and consequently when the last window goes
away the show desktop entry is added again.

There is a not considered corner case: if there is no desktop
window, the show desktop functionality is not available and
the behavior is unfortunately undefined. It is a corner case
as we can expect that there is always a desktop window when
using KWin. E.g. there is either Plasma Desktop or Netbook
and on PA there is always at least one window.

BUG: 260938
FIXED-IN: 4.9.0
REVIEW: 104379