Implement multi-component selection, move, and delete.

[dbrnz]

Keep a set of selected components in the editor:

• command-click will add/remove components from this set.
• shift-click on a component will select all components in the bounding box formed by the first selected component and the one shift-clicked.

Moving a component in the set moves all selected components along with the connections between them.

Delete will delete all selected components and their connections.