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.
Edited by Vlad Zahorodnii