This protocol supersedes the internal gtk_text_input protocol that
was in place. Functionally it is very similar, with just some more
verbosity in both ways (text_change_cause, .done event), and some
improvements wrt the pre-edit text styling.
If text_input_enable() is called when there no active IM (eg. running plain
mutter), some ClutterInputFocus method calls that are not allowed while
unfocused will end up called, triggering critical warnings.
If there is no IM return early here, all other calls are superfluous then.
This is the implementation of the internal text-input protocol that will
be used to communicate IMs (to be implemented by gnome-shell) with clients.
The text_input protocol has its own focus expressed through enter/leave
events, that will typically follow the keyboard's.
The client will be able to communicate its current status (eg. focus state,
cursor rectangle in surface coordinates, text surrounding the cursor
position, ...) and will receive commands from the compositor (eg. preedit
text, committing a string, ...).
Whenever there is an active input method, the compositor will route key
events directly through it. The client will not receive wl_keyboard
events if the event is consumed by the IM.
