Remove unused parameters

These underscores are unnecessary and cause warnings.
Change property setter to private on sealed class

A protected member does not make sense on a sealed class.
Add new keyword to interface members

These properties already exist on an interface that these interfaces
implement. The _new_ keyword is needed.
Properyly implement the interface

The autogenerated fields of these event properties were never used which
caused warnings.
Merge pull request #295 from reactiveui/warnings-fixes

Properyly implement the interface