0.0.7 server media rendering

Created by: bmarv

The created client-recordings can now be merged by the server via host-interaction. The host can trigger now not only the broadcast -start and -stop messages, but can also prepare the videos received by the server and also merge the videos to one output file. The Preparation of the videos means, that the file-resolutions will be cropped to a predefined value, by default 480p for both height and width, and the calculation of the client-video-tiles and the ffmpeg-command for merging the videos will be prepared. Merging, as well as Preparation, are compute-intensive processes. Merging the videos needs ffmpeg pre-installed to run bugfree on linux-machines. Osx is not supported any more as the server. The compability of the PR for 0.0.6 for the client-side (host, client) remains the same.