key-model: Use key-released instead of key-clicked to hide subkeys

This allows users to insert subkeys with a single button press/release
pair by pressing on the main key, then moving and releasing while
hovering the subkey.

Also, emit key-released instead of key-clicked on the main key since
that is a better model of what is really happening.

https://bugzilla.gnome.org/show_bug.cgi?id=688656