Have the default scene renderer show a wireframe on the children of selected nodes.
Improve the hierarchy panel in the scene editor.

With this commit, all of the selection functionality is present except for shift selecting.
Fix a recursion bug when trying to attach an ancestor scene node as a child.
Fix a crash when deleting a scene node from a scene when it has children.
A few improvements to the hierarchy panel in the scene editor to prevent creating redundant undo/redo points.
Add the shell ScriptComponent class in preparation for scene node scripting.