modify shadow keymap

[Antoine Martin]

At present, shadow servers leave the keymap completely untouched and try their best to find a matching keycode when we receive a key event from the client (see ticket:2301#comment:35).
We could add the option of re-configuring the shadow server's keymap to have the keys the client is likely to need.

ie: with a shadow server configured with 'us' layout and a client connecting with a 'gr' layout could use a 'us, gr' layout and make all the keys accessible.