fix showing desktop state

Fix for kwin bug 442044.

Relates to !1430 (merged) (integrated to avoid merge conflicts)

Implementation Details

Layers are divided now in 2 states:

1. normal
2. showing-desktop

• Every window type not explicitly handled in one of the two states is valid in both states (fallthrough part).
• Every unhandled window type results in NormalLayer

Questions:

• Should "fullscreen" windows allow showing-desktop? (currently they do)
• Do we have a test that can be used (adjusted) to verify behaviour is as expected?