x11: Emulate XKB's detectable key auto-repeat

x11: Emulate XKB's detectable key auto-repeat

Old-style X11 terminals require that even modern X11 send KeyPress
and KeyRelease pairs when auto-repeating. For this reason modern(-ish)
API like XKB has a way to detect auto-repeat and do a single KeyRelease
at the end of a KeyPress sequence.

The newly added check emulates XKB's detectable auto-repeat by peeking
the next event after a KeyRelease and checking if it's a KeyPress for
the same key and timestamp - and then ignoring the KeyRelease if it
matches.