instantmessages: Support multi payload messaging

 * A new signal to receive multiple payloads
 * Send the message as both html and text
 * Parse MIME format to extract all payloads
 * Send messages with multiple payloads

WARNING: This commit break the API

To restore the old behavior, users of sendTextMessage should
use the "text/plain" MIME type.

Refs #77651

Change-Id: Ic20c0cea48ba5d7ec625e69cd87eeffc1fdbe759