Compose and dead-keys support in libxkbcommon

[Ran Benita]

These patches add support for compose sequences/dead keys to
libxkbcommon, in a small mostly-independent module, xkbcommon-compose.
A while ago we decided this is a worthwhile addition, particulary for
applications which use libxkbcommon without an input method.

I'd appreciate any comments on the API. The implementation is mostly
included for reference if someone's interested.

You can also find the patches here (with some boring preperatory
commits):
https://github.com/xkbcommon/libxkbcommon/tree/compose

Thanks,
Ran