Made connecting of nodes interruptible 

I needed an option to interrupt the possible connection of two node connectors.

I implemented this through a new event `ConnectionAdding`, which is fired whenever a connector is picked as a candidate for possible connection. The overlaying application can interrupt this connection through the `Cancel` property of the `EventArgs` passed with the event.

I also introduced a new `RenderState` flag for this (to signal a forbidden connection), don't know if I implemented that the right way.