Tab pill framing and selection improvements.

Continuing work on getting the tab pills to get framed properly.
The TabInteraction now properly frames the pills.
Also the pills are now centered instead of being left aligned.

The Node and its derived classes' constructors will no longer
take a group argument. Instead the Group class class will automatically
call the Node::set_group method in order to record the back reference
on the Node.  This also gives us a single entry point for any logic
that need to happen when the node is added to a new group.

However there will likely need to be other logic for updating any
other additional spokes.