Add ability to specify a chat for sending track files

[therepanic]

In the current implementation, we strictly select the “Saved Messages” chat. I think it would be cool if we could somehow specify a different chat ourselves. This simplifies things, as we could put tracks in a specific chat and throw it into the archive, so we wouldn't only see it in Saved Messages, which is flexible.

I'm not entirely sure how best and easiest to specify this chat to the user and whether it's possible to do it simply. If it can't be done simply, then this issue will have to be closed; the most important thing is that it's simple.