Improve directory thumbnail sequences

Depends on !71 (merged).

This does 2 things:

1. When a directory has <= 4 children with valid thumbnails, it uses sequenceIndex() to animate the thumbnails of the children themselves. For > 4 children it keeps the old behavior of switching between different "pages" of 4 children each.

2. It adds a new custom entry "X-KDE-Directory-Thumbs" for Desktop Entry .directory files which can be used to include only certain children of the directory in the directory thumbnail. This is useful e.g. when the directory holds a video file and some additional metadata files for it, but the user doesn't want these metadata files to "crowd" the directory thumbnail, which can make the directory harder to recognize at a glance.

Example:

thumbnail-dir-sequence-improvements-2021-02-20_18.54.48