Start of a virtual keyboard input_server add-on

Done during GCI 2014.

Some files copied from the Keymap preferences.
virtualkeyboard: Add input_server device add-on code
virtualkeyboard: some fixes

Add initialization helpers.
virtualkeyboard: fixes; input_server now displays keyboard
virtualkeyboard: fix input_server event loop crash
virtualkeyboard: add TODO list
virtualkeyboard: add TRACEing and make the keyboard work
virtualkeyboard: Create README.md
virtualkeyboard: use US international keys for now

Update TODO list.
virtualkeyboard: add to the build
virtualkeyboard: fixed restart crash
virtualkeyboard: code style fixes
virtualkeyboard: mention needed code factorisation in TODO
virtualkeyboard: more TODO
AboutSystem: add Freeman Lou to the contributors

I'm not sure which is first name and which is last though...