Marble: Add dialog in case no avconv is installed.
completed by: Ilya Kowalewski
mentors: Dennis Nienhüser, shentey, Sanjiban Bairagya, Torsten Rahn
Marble has got a video capture feature. However we don't handle the case well, where no avconv/ffmpeg is installed.
Please add a dialog to Marble which pops up if no avconv is installed and which informs the user about the problem.