Refactor UI files

[nourgaser]

The current UI-related files are very unorganized.

• [x] Move the 'Resources' directory in 'UI/Resources' to the main Resources directory.
• [ ] Refactor USS files; split large files into appropriate directories and smaller files.
• Refactor CS files:
  • [ ] split large files into appropriate directories and smaller files.
  • [ ] move all in-line lambda subscriptions into named 'Handler' methods.

[nourgaser]

Addressed partially in 2c57728