* Add greeter keyboard and user selection
Fixes#37Fixes#38
* Support switching users and add tooltips to icon buttons
* Implement switching users
* Implement keyboard layout switching
* Ensure that user's xkb_config is used
Logins seem to spuriously fail with both correct or incorrect passwords.
The failure is not related to the password. `cosmic-greeter` hangs but
the GUI still works. The password text area disappears as well.
I traced this issue down to the socket. It seems like accessing the
socket deadlocks with one thread waiting for the socket to become
readable while another waits for it to become writable.
Switching to `greet-ipc`'s `TokioCodec` and adding a lock to the socket
seems to have fixed this issue. I successfully logged in and inputted
incorrect passwords consecutively without experiencing a deadlock.
