SMTK&CMB now has a concept of active model. Loading, switching and
closing model(s)/data would reset the active model correspondingly.
A singleton class(qtActiveObjects) is used to store active model.
subPhraseGenerator would also keep a copy of it.
Only active model can be selected by rubber band. And only active model
would be expanded in model tree. Attribute panel would only list active
model's entities. Check CMB issue #140 for detail.