Stop thumbnailing when playing a video

When Elisa user goes the first time in a movie directory, Gstreamer builds thumbnails for each file visible in the list.
If the user starts playing a video, the thumbnailing is not stopped. Consequences are :

- Lot of CPU is used when playback start (by the thumbnailing and the playback)
- The video is slow to start
- If Elisa is using a hardware decoder, the thumbnailing process can use it exclusively. the video will use a software decoder if available.

The solution could be to suspend the thumbnailing when a playback starts.