xwayland: Add xdg-system-bell support
For the Wayland compositors who do not implement XkbBellNotifyMask but support the Wayland protocol xdg-system-bell, use that to ring the bell.
That might also allow for simplifications in Wayland compositors who do not need to support Xorg anymore, they could drop their code that implements the XkbBell presumably, a this would now be going through the xdg-system-bell with Xwayland.