keysym: add function to test if a keysym is for a modifier

Needed for compose.

Signed-off-by: Ran Benita <ran234@gmail.com>
compose: add xkbcommon-compose

WIP

Signed-off-by: Ran Benita <ran234@gmail.com>
compose: add test

WIP

Signed-off-by: Ran Benita <ran234@gmail.com>
compose: add compose support to test/interactive-evdev

Definitely WIP (need to think how to integrate).

To try, do e.g.:
sudo ./test/interactive-evdev -l us -v intl -o compose:ralt -d

Signed-off-by: Ran Benita <ran234@gmail.com>