Add ColorBlending module for multi-image blending support

[izkgao]

Description

Closes #160.

Need frontend PR #2759 to be merged first. (merged)

This PR adds color blending support and refactors the session/image APIs around a shared "image-view item" model so frame-backed images and color blendings can be handled consistently.

What is implemented

• Add a new ColorBlending API with Layer objects for:
  • listing, adding, replacing, deleting, and reordering layers
  • preserving per-layer alpha during reordering/subsetting
  • setting layer alpha and colormaps
  • toggling raster/contour/vector visibility
• Introduce ImageBase as the shared abstraction for image-view items and move common behaviors such as make_active() and image_view_order onto the base class
• Update Image to align with the image-view-item model:
  • use file_id terminology consistently
  • expose richer __repr__ output
  • share activation and image-list-order behavior through ImageBase
• Extend Session with heterogeneous image-view-item support:
  • image_list() now returns both Image and ColorBlending
  • add _find_image_view_order()
  • replace the old image lookup helper with keyword-only image_by_id(image_view_order=..., file_id=..., color_blending_id=...)
  • replace active_frame() with active_image()
  • add create_color_blending() and open_as_color_blending()
  • add cached carta_version and include it in Session.__repr__ when available
  • update set_cursor() to use a better API
• Add new constants for ImageType and ColormapSet
• Update documentation and quickstart examples to cover:
  • heterogeneous image lists
  • image-view order
  • color blending workflows

Checklist

• Add code examples in documentation
• Add tests

[confluence]

I'm happy with these changes; I have a few more minor suggestions and a question.

[confluence]

One more issue with fetching lists -- this is the last thing; everything else looks good!

[confluence]

One more set of comments about the list code.

Also, a general comment about code style: because comprehensions are supposed to be compact, I favour using short iterator variable names rather than the long descriptive names that would be more appropriate in a loop. So e.g. it would be fine to write [i for i in some_list if i > something].

(If you prefer to use long names everywhere because of a linting tool, that's fine.)

[confluence]

I'm happy to merge this with @kswang1029's approval. I will review the remaining PRs once they're updated after the merge.