We need more robust user-edit signals from items in a layer

[cudmore]

As the user interacts with "layers", we can receive emitted events such as:

• inserting (layer)
• inserted (layer)
• removing (layer)
• removed (layer)

We need similar events for "items" in points and shapes layers (maybe labels layers).

This will allow plugins to be more interactive. Currently, plugins that interact with a layers "items" behave more like "batch" scripts and do not take into account edits made to the layer such as "items" added and deleted. Napari needs to emit a full set of editing signals such that plugins can easily implement a CRUD interface to create, read, update, and delete.

Thing like:

• 'item about to be added'
• 'item about to be deleted'.

Useful outcomes of this king of (proposed) event system are that analysis pipelines can:

1. Update in response to edits
2. Reject edits if the state of the plugin should not allow a given edit. For example, an event like "item about to be added" can be rejected.
3. We can implement "undo" for edits such ass (add, delete, move). SUPER USEFULL for NAPARi !!!

Proposal

For each layer, we need (add, delete, select) events:

• add

  • adding: about to add **
  • added: have been added

• delete

  • deleting: about to delete **
  • deleted: have been deleted

• select

  • selecting: about to select **
  • selected: have been selected

** plugin can reject add, delete, select if its internal state should not allow it.

Notes

This is where vispy responds to highlight event

# connects to signal
self.layer.events.highlight.connect(self._on_highlight_change)

# respond to signal
napari._vispy.layers.points.VispyPointsLayer._on_highlight_change