input-method-v3: add context_update event
Motivation
This event needed when input method want change their context certain situation
Example
Hangul preedit state need to be commited when focus-out or click
wayland/wayland-protocols#40 (comment 863260)
Immediate
It should be immediate because this update may conflict other buffered events
ex) context_update(focus_out) + deactivate
input method can't update context because it's already deactivated