Add support for HiDPi use case

Hidpi where the logical size does not correspond to the pixel on screen, needs appropriate thumbnails.

The proposed spec evolution specifies that Hidpi thumbnails might be generated in separate folders with "@2x" postfix added compared to the normal thumbnail folder. A special case can be used by the implementation when thumbnails are not influenced by scale factor but only by size, in such cases we can avoid storing twice images, i.e using "large" folder when we look for thumbnails scale independent for "normal@2x".

This was first proposed for discussion on xdg ML https://lists.freedesktop.org/archives/xdg/2020-May/014316.html

An implementation is proposed for KDE Frameworks https://phabricator.kde.org/D29397