Switched the default VNC handlers to the external variants

Ignore-this: e6912eee84b235da15cf8de578eae50b

darcs-hash:20140422060507-b22ec-efb97c3f2a32d3b371c1f07e414c7727bec11243
Rewrote the ScreenSharingHandler using abstract base classes

Ignore-this: 34880350d5534b8cfe030cee5a81985f

darcs-hash:20140423165403-b22ec-2cda7114f52e922d5564e2fab587a5d6afca6349