Fix streamer2

[hoffie]

This PR attempts to fix the QProcess-based ffmpeg invocation:

• It adds proper logging on ffmpeg exits by adding a ::finished handler and upping the ffmpeg loglevel so that something sane is output.
• It stops adding "ffmpeg" as the first argument as this seems to be done implicitly by QProcess (spotted using strace)
• It adds support for parsing space-delimited multiple arguments for ffmpeg. Otherwise, only a single argument would be supported and things like --streamto "-f mp3 /tmp/foo" would break.

Note: The crazy override for the ::finished handler is described here: https://doc.qt.io/qt-5/qprocess.html#finished

cc @npostavs @dingodoppelt

[npostavs]

• It stops adding "ffmpeg" as the first argument as this seems to be done implicitly by QProcess (spotted using strace)

Aha!