hlscmafsink/cmafmux: Add opus support, allow starting a new playlist without changing playback state

This MR brings two small additions to hlscmafsink and cmafmux:

1. Adds Opus support (gst-play-1.0 and Chrome seem to play the resulting output fine, mp4box -info doesn't complain about anything either)
2. Adds signals to allow for closing playlists/changing output location on the fly. Previously, to change the output directory, you had to change the playback state to READY, modify the properties and go back to PLAYING. Now, you can just change the properties and send the finish-playlist signal, which will close the old playlist and start a new one at the correct location. For this, a send-headers signal is added to cmafmux to make sure it'll re-send init headers for the new playlist.

(2) is nice to have if we want to accomplish gapless switching between directories. I implemented it in a very simple way, which hopefully won't break anything, at least to my knownledge it doesn't.