keyboard: Write the keyboard model and layout settings to the root mount point.

This is implemented as a new SetKeyboardLayout job that does the work.

Portions of the code are adapted from systemd-localed's source code,
which is under a compatible license (LGPLv2.1+, can be converted to our
GPLv3+ license). I ported it from C to to C++/Qt and made some minor
tweaks to the mapping logic (from X11 to vconsole layouts) though.

Fixes #31.

Tested on a Fedora Remix (Kannolo 21) with the default module settings
(finds the converted X11 keymaps for the virtual console) and with
convertedKeymapPath: "" (does the legacy keymap conversion as expected).
OK, I can pass the options directly rather than through the GlobalStorage if you think it's cleaner.