xwayland: Only update keyboard modifers for XWayland's keys
KeyboardInterface is a multiplexer, it has a global state to kwin that forwards events the single focussed window.
XWayland also forwards events to clients, but uses the keyboard interface. It has some overloads that take a specific client, this was used for key events but not modifiers.
The end result was not only that XWayland could miss a modifier update, but also that wayland clients would get modifier updates out of order. Key events must come first.