Add X11 keymap creation function

WIP
Needs --enable-x11 to configure for now.

TODO:
- Change asserts to proper tests.
- Add more asserts.
- Add error messages.
- Add device ID parameter.
- Add a real+vmod -> mask utility.
- Expand tests.
- Document.
- Commit message.

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