Use one-column model for thumbnails

Makes adding thumbnails simpler (and even faster) because there is only
one item per row.
Simplify ThumbnailView part 1
Simplify ThumbnailView part 2
Simplify ThumbnailView part 3
Simplify ThumbnailView part 4
Simplify ThumbnailView part 5
Simplify ThumbnailView part 6
Save a copy of the model pointer to ease the usage
Get rid of redundant includes
Trivial formatting fixes
Make thumbnail sort actions exclusively checkable
Sort thumbnails initially in ascending time order