* Added preview generation fallback feature.
When a preview generation fails (often for wmv/avi files), the new code tries with less stricted (no xerror) and more time consuming options (slow+fast seek).
Fix a minor issue when stash downloads ffmpeg/ffprobe, but doesn't re-detect their paths.
* Make mutating metadata ops mutation
* Implement scene generate screenshot
* Remove fetch policy on metadata mutations
* Port UI changes to v2.5
* Set generated image in database