Permit multiple selection, which can be dragged as one unit.

Rather than being allowed to select only one node, edge, or control
point, it is now possible to add multiple items into the selection by
holding down the Meta key when clicking an entity. (Meta = Cmd
(helicopter) key on MacOS, probably either Alt or Box on Windows.)
The Meta-click acts as a toggle, so you can remove items from
the selection also.

The multiple selection can be dragged to new positions on the canvas,
just like a single selection. However, because of this, it only makes
sense to allow nodes and control points within a multiple selection. An
edge has no location independent of the nodes/controls, so cannot itself
be moved.