Add documentation for image loading and icon exchange approaches

Adds docs/Image_Loading.md covering:
- JFace image infrastructure (ImageDescriptor, ImageRegistry, JFaceResources)
- Workbench image infrastructure (WorkbenchImages, ISharedImages)
- End-to-end image loading flow
- Existing CSS image support
- Analysis of approaches for CSS/theme-based icon exchange:
  - Equinox Transforms Hook (iconpack example)
  - URLImageDescriptor interception (#14)
  - Refined IImageURLModifier strategy interface

Author: vogella