Region editor wishlist

[keflavich]

Followup to #317

• Each region in a list should be editable
• Activate region selector on click
• Allow multiple region selection / deselection
• Allow bulk region movement

[keflavich]

#317 is merged. Todo items from that:

• Expand approach to other region types
• Investigate how to deal with Sky regions and WCSAxes
• Improve Matplotlib selectors to support e.g. rotation (could potentially be done as part of another PR since we can just raise a NotImplementedError in the mean time)
• Documentation

[keflavich]

Have we solved:

• Create a new region by clicking (region initialization)

[keflavich]

Related Matplotlib PRs that enable some of the above:

Rotation:
matplotlib/matplotlib#19853
matplotlib/matplotlib#19864

Selection:
matplotlib/matplotlib#19657

Polygon vertex addition/removal:
matplotlib/matplotlib#19660

[larrybradley]

The current status of outstanding work on interactive regions is captured in #391.