Add support for audio/multimedia messages

From @debacle user on muc: Is support for voice messages (or A/V messages) planned? I.e. press a button, record a message in some "good" file format and codec (e.g. Opus), press a button and HTTP up is the message. For receiving, there must be a nice audio or A/V player widget, of course, which is probably available in the Qt universe. [12:21:40] This feature would be cool, because:

1. for some people it's a good-enough replacement for A/V calls (Jingle etc.)
2. it's much, much simpler than Jingle
3. audio/video quality might even be better