Port away from deprecated QDesktopWidget functions & disable missing OSD settings functionalities on Wayland.

Port away from deprecated QDesktopWidget functions, following the instructions on https://doc.qt.io/qt-5/qdesktopwidget-obsolete.html . It turned out that neither OSD positioning, screen selection nor translucency work at the moment (or probably ever have worked) on Wayland. Restoring these functionalities on Wayland won't probably be straightforward, so I suppose it makes sense to just disable/hide non-functioning OSD settings there for now.