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).
Is this file possibly useful to other modules too? If not, the path can be passed in the ctor of SetKeyboardLayoutJob instead of GlobalStorage.