modify shadow keymap

[totaam]

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 #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.

[totaam]

Done for X11 shadow servers only in the commits above. It was trivial as the code just needed hooking up.
(MacOS and MS Windows servers would require key mapping setup code - which does not exist yet)

To enable, start the shadow session using:

xpra shadow --keyboard-layout=auto

(the default value for keyboard layout is unset)