Improve Course Select Loading

[ethanglide]

• Thumbnail loading done on romfs path set
  • Better choice now that texture loading is done in the background
  • Does not add significant time to start up (0.3s - 1s at most)
  • No more hanging on world select, improved performance
  • Thumbnails are likely fully loaded by the time the user opens course select, but will just be black rectangles until then
• Fix romfs path tampering crash
  • There was a crash that would occur if a user tampered with their romfs path outside of the program and had opened a course before
  • Certain things like the paramDB and latestCourse will now only load if the romfs path is valid
  • If the saved romfs path is no longer valid on startup, the preferences window will be open so the user can reset it

[KillzXGaming]

Seems good to me