feat(ui): add thumbnail caching

[CyanVoxel]

Summary

This PR adds the ability for file thumbnails to be cached inside a new thumbs folder within a library's .TagStudio directory. This cache used when possible to quickly update the thumbnails shown in the preview grid. Images in the Preview Panel still get rendered directly from their source files for maximum resolution. The main features of this process include:

• Cached thumbnails for files based on the filename and modification date of the files
• A total size limit for the cache (TODO: Make this user-configurable) where the earliest cached files are deleted when reached
• A menu option to clear the cache (TODO: Move this to a future settings screen)

Known Issues

• Audio waveform thumbnails no longer show their intended bar count due to always being passed a consistent size argument
• Every now and again I get a reproducible segfault-like crash with no traceback when casually using the program. I assume it's related to this PR but can't verify

Additional Considerations (TODO)

• Users should be able to configure where the thumbnail cache is stored
• Users should be able to specify the resolution and filetype of the cached thumbnails
• Users should be able to clear the cache from inside the program
• Users should be able to disable caching
• The program should be able to detect modified files and update the cache appropriately
• The cache should probably have a size limit
• There should be a way for the program to automatically clear old and/or orphaned thumbnails

Closes #104, Closes #345

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NOTE: This PR is currently awaiting the merging of #647 in order to add the thumbnail cache options to the new settings config.