Refactor debrief: known issues and potential improvements

[C-Achard]

This is meant to help list potential future additions and fixes to be made at a glance.

Potential features

• Skeleton display, see Seb/display skeleton #92 and Add option to visualize skeletons (edges connecting keypoints) #5
• Undo, Ctrl+Z
• Project manager GUI showing project files at a glance and providing quick actions
• Clustering, see clustering in napari-DLC GUI #43 and clustering feature in napari deeplabcut #38
• Frame removal utility for removing unwanted extracted frames
• Built-in evaluation tools for machine labels

Known issues

Bugs

• The color scheme list highlight color does not match the actual keypoint colors when loading a default config, see Update ClickableLabel in color scheme reference: don't use colored text #206
• Stray Qt pop-ups when opening, likely due to layout addition ordering. This might also delay opening
• If no configuration file is present during video frame extraction, ask the user for one and ideally save relative to it
• The body-part and individual toggle menu is sometimes incorrectly shown on startup
• If KeypointControls is added by drag and drop, another widget instance can still be opened
• Moving a keypoint on frame t and then going to t+1 in loop mode does not add a keypoint and immediately skips to the next frame

For future PRs

• Further separate the remaining responsibilities in _widgets.py
  • Initial separation started in Plugin refactor [8]: Improve layer management/lifecycle #186
• Fix or silence the warning from the napari Qt progress bar about constrained layout not being applied because axes sizes collapsed to zero
• Add a hard guard against loading multiple datasets at once, or otherwise make this workflow safe
• Add more ergonomic labeling controls, such as A and D to navigate frames, W and S to switch keypoints, and Shift or Space for special actions or saving -> Add more ergonomic keybindings for navigation #225
• Give machine-label layers visually distinct markers -> Add centralized layer display settings, make machine labels visually distinct #216
• Disable or patch the 5 shortcut for the transform tool and show a warning when appropriate
• Further centralize logic, probably around schemas that guarantee their own validity and provide relevant operations
  • Centralize I/O behavior in particular
  • Use this foundation to simplify future keypoint-handling refactors
  • See https://github.com/DeepLabCut/napari-deeplabcut/milestone/4
• Reduce the heavy dependency of all I/O operations on pandas
• Relax dependency version pins, including support for newer napari versions -> Relax napari version pin #217

Miscellaneous

• Move the Pydantic schemas from config/models.py into core
• Namespace plugin metadata in layers under a napari-dlc key instead of using a flat dictionary, to improve interoperability with other plugins and metadata edits