Add Editor::onDidMoveCursor
Remove ‘cursors-moved’ event

It event was not document only being used in EditorComponent. Due to
our batching strategy, it’s fine to respond to individual
::onDidMoveCursor events.
Add Editor::onDidAddCursor
Add Editor::onDidRemoveCursor
Eliminate Editor::onDidMoveCursor

If you want to know if a cursor moved, subscribe to the cursor.
Add Editor::onDidAdd/RemoveSelection

Also update EditorComponent to use the new ::onDidAddSelection method.