terminal: add xkb-based keyboard devices to idev

terminal: add xkb-based keyboard devices to idev

The idev-keyboard object provides keyboard devices to the idev interface.
It uses libxkbcommon to provide proper keymap support.

So far, the keyboard implementation is pretty straightforward with one
keyboard device per matching evdev element. We feed everything into the
system keymap and provide proper high-level keyboard events to the
application. Compose-features and IM need to be added later.