Conditional dependency btw libavcodec-ffmpeg56 and libavcodec-ffmpeg-extra56

On several Linux version (Mint 18 for example)  the package libavcodec-ffmpeg56 is replaced by libavcodec-ffmpeg-extra56.
To increase the compatibility I added an exception which define a conditional dependency between these two packages.

Revert "due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)"

This reverts commit ae1a2a53.

This reverts commit 08f7b3c1.

due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)