Window Manager: make sure mixing effects for minimizing and mapping work together

In particular, make sure that the combination of desktop-effects-minimize-effect 'fade' and desktop-effects-map-effect 'none' does not leave the restored window invisible.

This also takes care of the case where desktop effects are temporarily turned off when restoring a minimized window, which could leave the restored window in a bad state.

In addition, this makes the code indifferent to the particular panel layout that is in effect, paving the way for the panel rework that is long overdue.