Cache part thumbnails on disk

[bb1950328]

• Add setting to change maximum size of disk cache (there's already one for the memory cache). A value of 0B means that there should be no caching.
• There should be two options: 1 file per part/color and 1 file for the whole cache (archive or sqlite db)
• experiment with compression (png/jpeg and OpenGL texture compression)

[bb1950328]

13754 parts in the most recent LDraw release
that means 2.70GB of raw uncompressed images for 256x256 (10.81GB for 512x512 or 0.68GB for 128x128)
If we put all 256x images into one atlas, it would be sqrt(13754*256^2)=30023px large. Maybe we can use an ImGui font atlas for this...