Fix README, add RdpControl.OnClientAreaClicked event and fix InvalidActiveXStateException in RdpControl.Disconnect

[haefele]

Hey, just some small things I noticed while using the library.

• The README referenced your other package for FreeRDP in some places.
• I also noticed that there was no RdpControl.OnClientAreaClicked event, but all other events of the RdpClient are available through the RdpControl (Connected, Disconnected, etc.)
• And I also experienced a InvalidActiveXStateException in RdpControl.Disconnect() method when the underlying Ocx control was still NULL. The code in that method seems very defensive, so this additional check makes sense to avoid the exception.

Just some small things, but would be appreciated if you could merge it 😄

[StefanKoell]

Thanks for the PR!