Remove unnecessary include.
Display placeholder before thumbnails are created.
Don't try creating thumbnails for images not on disk.

This prevents the AsyncLoader (-> thumbnailViewer), and the automatic
search for videos without valid length/thumbnails from spawning
thumbnail jobs for images/videos that are not currently on disk.
Fix typo.
Make ImageManager cope with missing mplayer.
Remove unnecessary include.
Run without video thumbnail/length support when mplayer is missing.

When mplayer is missing, just show a messagebox advising the user to
install mplayer instead of bailing out.
When mplayer is not available, also don't spawn video-related background
jobs.

BUG: 311118
DuplicateMerger: select none by default.

Don't select any entry in the duplicate merger to avoid accidental
deletion of images.

BUG: 320471