- DrawablesSelection class now inherits from ObservableContainer.
- Calls to notifyObserver() inside of DrawablesSelection code have replaced by calls to notifyElementInsertion(), notifyElementDeletion(), etc.
- Removed the class InformerContainer.
- Added a new ContainerObserver class.
- Now the ObservableContainer class inherits from Observable, allowing it to be observed by both Observer and ContainerObserver instances.
- Multiple classes' base class has been changed to Observable or ObservableContainer.