terminal/idev: add compose-file support

Add support for compose files to idev-keyboard. This requires
libxkbcommon-0.5.0, which is pretty new, but should be fine.

We don't use the compose-files, yet. Further commits will put life into
them.
terminal/idev: use compose tables

Before forwarding keyboard events, feed them into possible compose tables.
This enables Compose-key and Dead-key features.

Few notes:
 * REPEAT events are never fed into compose tables. It just doesn't make
   sense and is usually not wanted. Compose-sequences are usually hard to
   remember and take time to type. Thus, the REPEAT event of the
   Compose-key itself would often cancel the compose sequence already.

 * Stop resolving symbols for UP events. Anything but keycodes is never
   associated to a physical key, but is a one-time action. There is
   nothing like UP events for key-symbols!

 * Cancel compose-sequences on Multi-Key UP. See the inline comment. We
   should make this configurable!