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).
> Space inside parentheses.

I thought I'd have that one nailed, but you found a place I missed. ;-)

> Also block opening goes on next line.

Oops. (I'm not used to that style.)

I'll fix the formatting (and hope I catch all places this time).