Bug 423541

This bug also extends into kio-extras and the thumbcreator implementations

 * If the thumbnail creation is cheap (such as text previews), you can set
 * \code
 * CacheThumbnail=false

Thumbnailers can opt out of caching but that seems more like a disadvantage than anything.

The only thumbnailer making use of this is the text thumbnailer, so we have this entire feature for a single use case.
What's more this kinda impairs making the directory thumbnailer cacheable (also see bug #423538). Directory thumbnails are composites of its content thumbnails. If some of its content can opt out of caching then the dir thumbnailer has a much harder time to determine if the content thumbnail changed compared to the previous thumbnailer run and so it has no option but to always re-composite itself even when the directory content hasn't changed.

I'm also not sure thumbnailing is ever really cheaper than loading that very same image from PNG. In particular since the very example of text thumbnailing is apparently also not nearly as cheap as one would think thanks to encoding detection being a pain https://phabricator.kde.org/D29381