ui: improve thumbnail grid browsing

[CyanVoxel]

Note

This PR is targeting the dev branch for release with Alpha v9.7.

Summary

(Builds off of #1539 + #1540) This PR adds several improvements and fixes to further polish the thumbnail grid:

• Holds rendered thumbnails for a split second to display them all at once instead of "popcorning" in
• Reuse page results to help enable very quick page flipping
• Fetch Entry IDs more efficiently from the DB in searches
• Fix inconsistent empty search behavior
• Use PIL's draft function to more quickly decode JPEGs for thumbnails and previews
• Reduce some hangups on app shutdown, notably with thumbnail renderers
• Fade in rendered thumbnails over loading icons (ones that don't make the batch display cutoff)
• Fix animation and sizing quirks with the banner
• Cache ItemThumb Qt paint objects
• Fix themed variants of thumbnails not caching separately
• Clear the current grid selection by clicking the thumb grid background

Progress on #1460.

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable