Issues in Workspace::lowerWindow()
There are several issues in Workspace::lowerWindow() described here: BUG: 478382 and here: BUG: 478383
This MR has 3 commits: first adds test for lowerWindow(), second fixes: BUG: 478382 (those problems only show themselves on X11 now, but seem "general"), last commit fixes wayland ( BUG: 478383).