#2 Multiple node selection

[Brendan Cleary]

Currently the user can only pick one node at a time; we
wish to modify the picking behaviour so that the user
can select multiple nodes simultaneously by holding
down a modifier key and clicking each node in turn.
Selected nodes should be displayed as they are
currently when selected, highlighted and their text set
to visible and surrounded by the highlight glyph. Also
we should modify the existing node selected listener
interface and node selected event to reflect the new
use case.