Add a few TypeForwardedTo attributes for types moved to System.dll
* System.Collections.ObjectModel/ObservableCollection.cs [NET_4_0]:
Forward type to System.dll.
* System.Collections.ObjectModel/ReadOnlyObservableCollection.cs: Likewise.
* System.Collections.Specialized/INotifyCollectionChanged.cs: Likewise.
* System.Collections.Specialized/NotifyCollectionChangedAction.cs: Likewise.
* System.Collections.Specialized/NotifyCollectionChangedEventArgs.cs: Likewise.
* System.Collections.Specialized/NotifyCollectionChangedEventHandler.cs:
Likewise.
svn path=/trunk/mcs/; revision=146678