Get rid of Model classes from CoreObject

[qmathe]

COCollection, COGroup, COContainer should probably all be discarded.

COTag, COLibrary and COBookmark should be moved into a new library that sits on top of CoreObject.

COTag and COLibrary needs to be rewritten to support managing a list of persistent roots without loading them.

Not sure what to do with COSmartGroup… Keep it in CoreObject as a conveniency or put it in a new model library. In addition to in-memory search against object graphs, COSmartGroup should probably support searching the store directly. On this topic, COQuery should also be discarded.