Draft: Use mimetype registries for preview settings instead of listing all available thumbnail plugins

Use registry names as depicted by IANA for preview settings.

These are the first halves of mimetypes, such as image/* and application/*. What this code basically does is use that first part as a wildcard to generate any thumbnails with any available thumbnailers as long as they're part of that registry family.

This results in more clean preview settings, with less potential duplicates:

Current	New

As you can see there's way more stuff in the old one, with things like "Images" and "JPEG Images". I do not think user is concerned with exact different mimetypes, they just want to toggle videos on or off for example.

If there is no any thumbnailer for a certain registry, then it's not shown in the list at all.

Note that inode registry has human readable name Folders since that's what our plugin is marked as.

The following MR's rely on this change

• Dolphin: system/dolphin!818 (closed)
• kio-extras: network/kio-extras!366 (closed)

Todos:

• filepicker does not have thumbnails loading anymore