xwayland: Emit Xwayland::started() after WM_S0 is claimed
Xwayland starts listening to -listenfd file descriptors after the WM_S0 selection is claimed. At the moment, it is claimed asynchronously by kwin. First, we create a dummy window and change one of its properties to get the timestamp. After the timestamp has been received, we actually call xcb_set_selection_owner().