Ground-up rewrite of the data model. Putting this in the ve2 directory for now so we still have the old code floating around.
Main changes so far in this rewrite:
* Renamed hasChildren() to canHaveChildren()
* Added canHaveGrandchildren()
* Added a new node type TwigNode that can have children but not grandchildren (so all of its children must be LeafNodes)
* Implemented push/pop/shift/unshift as wrappers around splice()
* Renamed getElementType() to getType(). Nodes now take a string as a type, and the element stuff is gone and won't be back
* Removed clearRoot(), replaced it with setRoot( this ) where needed
Change-Id: I23f3bb1b4a2473575e5446e87fdf17af107bacf6
