BugFix: non-draggable nodes break select/deselect

Selecting the root node broke selection/deselection and drag. It
appears that when you have a non-draggable node selected, the mouseup
event doesn't get called.

Fix appears to be ignore disabled/non-draggable nodes even if they are
selected.

Otherwise hide the root node, or prevent selection of disabled, or
undraggable nodes.