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).
keyboard: Coding style fixes.
keyboard: kbd-model-map: Add link to the original.

In particular, to the upstream git log for the file.
keyboard: Pass settings directly, not through GlobalStorage.

Pass the settings from keyboard.conf directly to the
SetKeyboardLayoutJob rather than through GlobalStorage.
keyboard: Allow absolute paths for the xOrgConfFileName setting.

This should ensure it works on all distributions.
Merge pull request #147 from kkofler/fix-issue-31

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