fix: stop ffmpeg cmd windows, refactor ffmpeg_checker

[CyanVoxel]

Summary

This PR fixes numerous issues with the ffmpeg_checker.py file. Fixes #854.

Changes

• Stop showing several CMD window popups on Windows builds when opening the "About TagStudio" screen
• Show correct FFmpeg and FFprobe version text, notably for nightly builds
• Refactor ffmpeg-checker.py:
  • Remove backend logic from this UI class
  • Remove duplicated logic
  • Only instantiate class if necessary, not every time on startup
  • In the version check (now moved to vendored/ffmpeg), use consistent logic and remove (suspiciously bizarre) flags and string processing steps
  • Add missing translation strings

Example of correct version readouts on Windows 10:

Tasks Completed

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