Make variants in bead plots collapsible

[ArtPoon]

Just an idea. Since we are having problems interpreting enormous beadplots, it might help to collapse descendant variants into a single ancestral line segment, kind of like a tabbed HTML interface

[ArtPoon]

Was also thinking about having beadplot get redrawn in response to user clicking on a particular variant or bead, so that its immediate ancestor and descendants ("siblings") come into view by reordering the tree.