Make show desktop mode hide windows
After porting the desktop background window to the layer-shell protocol, the desktop window is not raised up in the stack anymore when the showing desktop window is activated. The main reason to avoid this is to avoid fighting and overriding layer shell logic.
As another way to implement the show desktop mode, this change makes the workspace hide windows that don't belong to desktop.
It's a better solution for a couple of reasons: "keep above" and other overlay windows will be properly hidden and it lets us avoid touching the stacking order.
Edited by Vlad Zahorodnii