compositor-x11: Attempt to track state without XKB

compositor-x11: Attempt to track state without XKB

This isn't very pleasant, but it's pretty much the best we can do in the
absence of either XCB XKB support, or XCB XI2 bindings (argh!).

We get a state mask with most significant X11 events (which inexplicably
includes EnterNotify but not FocusIn), but unfortunately it's only a
single flat set of effective modifiers rather than the more granular
sets we want, so we still update the state with every key, but then also
use the core X11 state as a mask to make sure we don't get any stuck
modifiers.

Ugh.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>