Updated all FileSystemWatcher events to be virtual, to make them possibl...

...e to raise via mocks.

I realized that without making the events virtual there is no way for mocks to raise them.  While I'm not a fan of virtual events (as you can shoot yourself in the foot by overriding events) I could not figure out a better way to allow mocks to raise events without changing the `FileSystemWatcherBase` to be an interface methods publicly.