Linux thumbnailer support

Add initial support for thumbnails in Linux file managers.

Based on this mechanism: https://wiki.archlinux.org/title/File_manager_functionality#Use_PCManFM_to_get_thumbnails_for_other_file_types

Requires a manual update of the mime database as described here: https://wiki.archlinux.org/title/XDG_MIME_Applications#New_MIME_types

Some extensions required specific mime-type stuff to work, not sure if this is correct: gml, 3ds, obj

This also modifies the behavior without --output to not render and fails when data cannot be loaded.

• pcmanfm
• caja
• nemo
• pcmanfm-qt
• deepin: Unclear, may have some kind of API, will not do: https://github.com/exe-thumbnailer/exe-thumbnailer/issues/13
• dolphin: KDE has it's own API for thumbnail generation, will not do: https://api.kde.org/frameworks/kio/html/classThumbCreator.html
• konqueror: KDE has it's own API for thumbnail generation, will not do: https://api.kde.org/frameworks/kio/html/classThumbCreator.html
• nautilus: local bug, to test on others computer
• thunar (unstable for some reasons?)
• pantheon: do not work even with ffmpegthumbnailer
• peony: thumbnail system unknown
• qtfm: do not work even with ffmpegthumbnailer
• rox: do not work even with ffmpegthumbnailer