Fix unbind and rebind of event handlers

Fixed event handler problems that prevented enableCue() and disableCue() from working.

Use zDepth and zIndex for group depth

Use both zDepth and zIndex to determine top group when there are overlapping groups.

Allow hover over nodes and edges

Hovering over nodes and edges inside a group will no longer make the cue disappear.

Touch for expand/collapse

feature: touch works for expand and collapse
bugfix: if edge is over a cue, edge no longer gets selected

Multiple selected groups

Supports multiple selected groups and renders images better. Hovering is less buggy.