Add a standalone implementation of NotifyPropertyChanged class

[bramborman]

Using this we'd be able to use this class as a backing storage for classes that don't inherit from the NotifyPropertyChanged class. Could be static or non-abstract non-static class.

There'd have to be an option to release all the backed values to prevent memory leaks when the object is garbage collected. Cause of this it'd be better not to have one common static class but rather non-abstract non-static class whose instance could be stored inside the related object.