Model Collection view

[phongvis]

Add a view to manage models of interest with the following features:

• add models to the collection (e.g., brush models elsewhere and click on a button in this view)
• remove models (e.g., brush models in this view and click on a button to remove)
• save/load the collection to/from a json file

[phongvis]

add models to the collection (e.g., brush models elsewhere and click on a button in this view)

yes, but not work with aggregated models

save/load the collection to/from a json file

done