GUIDialog Python Proc fixes

[dpogue]

• Fixes an error when a PythonFileMod tries to set itself as the proc for a GUIDialog (which happens when a dialog is passed in as a Python attribute)
• Fixes a case where GUIDialogs would get added as pool objects to the scene node, but also (incorrectly) get attached as modifiers to a SceneObject. This can cause Uru to crash because things load in the wrong order (which is its own problem that I'm not spending more time debugging tonight)
• Allow creating node trees in GUI pages. Sometimes you want a PythonFileMod in a GUI page, or sometimes you want a ResponderModifier in a GUI page.
• Fix a case where all ATC anims were being listed as pool objects which is not necessary except in the case of avatar animations (and those should in future probably be special cased like GUI pages are)