Now addons can format messages with new slots accepted by
IM_MESSAGE_RECEIVED: "face_start," "face_length," and "face".
The first two deal with the position of the face-change by character
offset in the string, and the last being the face flag affected.
Base-level replacement of the Vision-derived text RunView― a couple
important features supported by the replacee haven't been implemented yet,
including:
* Right-click menu
* URL-highlighting/selection
* Emote support