keyboard: allow setting keymap directly

[nicolasavru]

Also make the Layout.0 public to allow constructing Layouts from u32s.

[Drakulix]

To be clear: I don't oppose this change.
But what is the use-case for it? It would be easier to review and judge if this should be part of the public api, if I would knew what exactly you do with it.

[nicolasavru]

A passthrough compositor. In the wayland client part of my code, I'm getting the keymap as a string/file descriptor and the layout index from update_modifiers (Smithay/client-toolkit#430 for that), and need to pass that information on to clients running against my compositor. So I have the full keymap file and can construct a Keymap, but I don't have the RMLVO string which is what compositors usually construct a keymap from.

[ids1024]

#750 has something similar. But I noticed the API is potentially unsound since xkb::Keymap is reference-counted, in a way that isn't thread safe. (See the unsafe Send impl in src/input/keyboard/mod.rs).

[nicolasavru]

I can modify this api to accept a String instead and construct the keymap inside set_keymap with Keymap::new_from_string (that's how I'm constructing the Keymap outside the function currently). I'll send a pr for that in a few min.

[nicolasavru]

#1221 for that change. Thanks for catching that.