extension methods

- ADD: StartNewTask() extension methods
fixes

- FIX: result of SetProperty method in NotificationObjectBase class
StartNewTask

- CHG: swapped parameters of StartNewTaskAction<T> and
StartNewTaskFunc<T, R> delegates
IValueConverter and fixes

- FIX: NullReferenceExceptions
- FIX: file name typos
- ADD: strong typed IValueConverter base classes