backends/x11: Implement own keyboard interception

At the moment, the keyboard interception code in the effects system
relies on Qt code processing key events. However, since QDesktopWidget
is removed in Qt 6, this is a blocker for Qt 6 port.

This change ports the X11 backend to private xkb keymap as indicates in
the todo comment. It allows us to drop the last QDesktopWidget usage.