[Bug] Unify selection events

[miroiu]

Two SelectionChanged events are triggered when calling NodifyEditor.EndSelecting if the selection contains both connections and item containers.

To Reproduce

1. Select at least one connection and one item container
2. Start and complete a new selection in the editor by clicking on empty space

Expected behavior
I would expect a single (custom?) SelectionChanged event to be raised, containing information about the selected and deselected connections and items.

Additional context
Implementing correct behavior for selection undo/redo may be challenging without this improvement.