Feature: Improved thumbnail placeholder performance

[workbysaran]

Resolved / Related Issues

When browsing folders, items previously displayed a placeholder while thumbnails loaded asynchronously. This change implements in-memory file/folder icon caching to show icons immediately during enumeration instead of the placeholder. Thumbnails still load and replace the icons as before.

For #4180

Steps used to test these changes

1. Navigate to a folder with many files and verify the standard icon is shown and subsequently replced with the actual thumbnail of the file/folder.
2. Navigate to a folder containing special folders (Downloads, Documents, Pictures) and verify the same.
3. Navigate to a folder with executables and verify the same.