Fixed: Memory leak in CPTableView, CPScrollView

Prevously, when removing a CPTableView or a CPScrollView, the CPNotificationCenter kept a reference of these observers in the notification center.

Now, the CPNotificationCenter does only have a observer when necessary.
This is still used by `CPTableHeaderView` in order to collect multiple resizing events when the user is changing a column width. Removing this method will cause a crash there.