Fixed: memory leaks with CPNotifications

Previously, when removing a view, Cappuccino didn't clean the notification center. The notification center kept in reference old views.

This PR fix this issue. When a CPView is added to a view, the methods _removeObservers and _addObservers are called. In these both methods we remove and add the observer to the notification center if needed. _removeObservers and _addObservers are called for the view and its subviews. These both methods are called through the method viewWillMoveToSuperview.

When a CPView is removed, we only call the method _removeObservers.
When a CPWindow is closed, we call the method _removeObservers on its contentView.
When a CPWindow is about to be opened, we call the method _removeObservers and _addObservers on its contentView.

Refs #1880
Refs #2024

Test app in Tests/Manual/AttachedSheet2/SheetWindowController.j
Fixed: make sure to remove observers when opening a CPWindow