linux/x11: handle XIM events sync to reduce lag (#12840)

This helps with the problem of keyboard input feeling laggy when the
event loop is under load.

What would previously happen is:

- N events from X11 arrive
- N events get forwarded to XIM
- N events are handled in N iterations of the event loop (sadly, yes: we
only seem to be getting back one `ClientMessage` per poll from XCB
connection)
- Each event is pushed into the channel
- N event loop iterations are needed to get the events off the channel
and handle them

With this change, we get rid of the last 2 steps: instead of pushing the
event onto a channel, we store it on the XIM handler itself, and then
work it off synchronously.

Usually one shouldn't block the event loop, but I think in this case -
user input! - it's better to handle the events directly instead of
re-enqueuing them again in a channel, where they can accumulate and need
multiple iterations of the loop to be worked off.

This does *not* fix the problem of input feeling choppy/slower when the
system is under load, but it makes the behavior now feel exactly the
same as when XIM is disabled.

I also think the code is easier to understand since it's more
straightforward.

Release Notes:

- N/A