scanner-utils: add helper for hex string escape

Like the already existing oct.

Signed-off-by: Ran Benita <ran234@gmail.com>
scanner-utils: add helper for appending an entire string

Signed-off-by: Ran Benita <ran234@gmail.com>
keysym: add function to test if a keysym is for a modifier

Needed for compose.

Signed-off-by: Ran Benita <ran234@gmail.com>
Add utf8.{c,h} for common UTF-8 util functions

We need to validate some UTF-8.

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>