[Refactor] type fixes and minor improvements to preview_thumb.py

[VasigaranAndAngel]

Summary

• Minor type fixes
• Stop preview with setting media player's source to an empty QUrl.
• Removed placeholder.mp4

Tasks Completed

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

[CyanVoxel]

Did you test deleting video files that the media player is currently playing? That's what the placeholder.mp4 was there for before

[VasigaranAndAngel]

Did you test deleting video files that the media player is currently playing? That's what the placeholder.mp4 was there for before

yes i did test. and according to doc setting an empty QUrl as source will release the loaded file.

[CyanVoxel]

Sorry for the delay on getting this merged!
I've tested this across macOS, Windows, SSDs, and HDDs and it seems like changes to the media player and its interactions with resource locking are safe. The rest of the type hint fixes are appreciated as well!