Draft: BUG: 440540 Fix Toggle Raise and Lower in presence of Keep Above windows on the same desktop - v2

Fix BUG: 440540, this is a solution I consider superior to e6f53d93. User experience is better and the patch is leaner and not scattered over several places. However it extends API of KWin::Workspace class, which breaks its ABI - and ABI of libkwin.so as an effect (but I don't think this is a problem, everything must be compiled from the same source anyway (?)).

Commit message:

Checking if a window is top-most is now limited to the same layer
windows only (i.e. the window that is top-most within its own layer is
"good enough" to be lowered by "Toggle raise and lower" action).

This approach is even better than the original implementation, because
in that implementation - in the presence of windows in higher layers -
windows that were already top-most within its own layer needed to be
"promoted" to quasi top-most state (by means of assigning them to
most_recently_raised field) when "Toggle raise and lower" was triggered
first time; this "promotion" was visually a no-op and user needed to
trigger "Toggle raise and lower" action again to make that window
actually get lowered. This quirky behaviour is eliminated with this
patch.