Draft: add keyboard config for X11

Marking as draft, because I would really like to use another approach with XCB_ variables, but couldn't get it to work. Using environment variables would let us use same settings in Wayland compositors as well. Also, that is a cleaner solution. I tried such approach with LXQt to no extent, so if anyone knows how to make it see XCB_DEFAULT_LAYOUT, XCB_DEFAULT_OPTIONS and XCB_DEFAULT_VARIANTS, please let me know.

This code adds /etc/X11/xorg.conf/99-keyboard.conf and sets secondary layout, as well as layout switcher combination.

The code itself is in working state, was tested against different edge cases and can be merged anytime.