Plasma Package support for desktop switcher layouts

The existing desktop switcher becomes the first available layout called
"informative". For both variants of desktop switchers a new config key is
introduced to define the desktop switcher layout.

Desktop layouts are installed into a different directory than window
switcher layout and use a different service type.

For the moment it's basically a hidden config option as there are no
further layouts yet.

BUG: 296068
FIXED-IN: 4.11
REVIEW: 110021