Improve compose sequence handling:
- Show preedit for compose seqences
- Support sequences of up to 20 code points
- Warn when ignoring Compose file features
- Support compose sequences producing multiple characters
- Support hex escapes
People might put all sorts of gunk in their .XCompose file, in
the hope that XLib makes sense of it. Even if we don't make sense
of it, we shouldn't abort, but instead ignore the lines we can't
understand. Pointed out in
https://bugzilla.redhat.com/show_bug.cgi?id=1301254
