Improve show desktop behavior

The current behavior of the "show desktop" shortcut (ctrl+alt+d) is quite confusing.

What it currently does is toggling a global "desktop mode", which is basically unnoticeable by the user, and minimizes or raises the windows according to that (mostly) internal flags. On the first toggle it's fine: it shows the desktop. However, if after entering desktop mode the user manually raises a window (alt+tab, clicking on the DE's taskbar, etc.), it does not leave the desktop mode, and a subsequent ctrl+alt+d will leave desktop mode, which will re-raise all previously minimized windows, whereas the user most likely expected to show the desktop again, having had a window on top of it previously.

IMO this behavior is very confusing, and I've have several other people confused by it. I also fail to see how useful that behavior is, form a user POV.

What I suggest is to instead show the desktop whenever a window is on top, and raise windows when none are. The patch here is an attempt at doing that.

Note that I also fail to see the use case for this "desktop mode" thing. I agree that it's great that windows have a "hidden because of showing desktop" flag, which allows to only re-raise windows that were minimized for showing the desktop, and leave manually minimized ones alone; but I don't really get the whole "showing desktop" Atom and all.