Projector: Add selection editor

[francoisluus]

In the Projector any point selections can be modified by toggling selection/deselection, which can be done for single points or a collection.

Demo here: http://tensorserve.com:6012

git clone -b projector-selection-editor https://github.com/francoisluus/tensorboard-supervise.git
cd tensorboard-supervise
git checkout 9d9a9658fa8ffbe29e55ed375f2abd97019f4591
bazel run tensorboard -- --logdir /home/emnist-2000 --host 0.0.0.0 --port 6012

Design and behavior

1. Uses toggle button for edit mode, like "Bounding box selection".
2. When edit mode is off Projector works exactly as before.
3. When edit mode is on, any existing point selection will toggle selection/deselection for any chosen points, even for multiple points now selected with bounding box.
4. Inspector panel neighbors list gets updated with appropriate distance calculated for any newly selected/added points, which are inserted correctly to keep distance sorting of neighbors list.
5. The main point, i.e. an initial point for which a neighborhood is calculated, cannot be deselected in edit mode.
6. Checks are in place to ensure no points are duplicated in the selection arrays.

Projector toolbar before:

Projector toolbar after:

[dsmilkov]

All looks great. I just left 1 optional comment regarding the text of the button.

[dsmilkov]

"Append to current selection" might be clearer, but I'm open to other suggestions if you find that confusing.

[francoisluus]

Appending/selection is one half of the functionality, with the other half being removal/deselection. "Add/remove in current selection" is closer to the functionality that needs to be conveyed, but perhaps that's too long hence "Edit current selection".

Granted, "Edit" may be too ambiguous/vague, and "Appending to current selection" could work where deselection is just a bonus with the main intent being additional selection.