Display multimedia messages: audio, video, stickers (gifs)

[kevinaboos]

Currently Robrix supports displaying text messages and static (non-moving/animated) image messages.

The Matrix spec supports multiple other kinds of multimedia messages, as defined here by the MessageType enum. This issue is primarily concerned with:

• Audio: the AudioMessageEventContent type
• Video: the VideoMessageEventContent.
• Stickers: the StickerEventContent type
  • This is separate from the other MessageType variants
  • Stickers can be gifs or other animated multimedia files, but they can also be regular static images. The specific type is given by StickerMediaSource::mimetype

Currently, this is blocked on Makepad adding support for video and audio playback. @joulei is currently working on it.