We may want to be able to enable/disable IME run-time, even though we
have received an ‘enter’ IME event.
This enables us to do that.
Also add functions to enable/disable IME on a per-terminal instance
basis.
A terminal may have multiple seats focusing it, and enabling/disabling
IME in a terminal instance enables/disables IME on all those seats.
Finally, the code to enable IME is simplified; the *only* surface that
can ever receive ‘enter’ IME events is the main grid. All other
surfaces are sub-surfaces, without their own keyboard focus.
This is done by allocating cells for the pre-edit text when receiving
the text-input::done() call, and populating them by converting the
utf-8 formatted pre-edit text to wchars.
We also convert the pre-edit cursor position to cell positions (it can
cover multiple cells).
When rendering, we simply render the pre-edit cells on-top off the
regular grid. While doing so, we also mark the underlying, “real”,
cells as dirty, to ensure they are re-rendered when the pre-edit text
is modified or removed.
