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.

BUG: 387593
BUG: 406101
BUG: 469827