#77 Key Events added to ZedGraphControl

[Tim Grudt]

Added the ZedKeyEventHandler delegate and KeyDownEvent and KeyUpEvent events to the ZedGraphControl class. These are modeled on the ZedMouseEventHandler delegate and mouse-related events like MouseDownEvent. These events are checked in ZedGraphControl.ZedGraphControl_KeyDown( ) and ZedGraphControl.ZedGraphControl_KeyUp( ) similar to the way the mouse events are checked in handlers like ZedGraphControl.ZedGraphControl_MouseDown( ). These additions allow users of ZedGraphControl to prevent the code in ZedGraphControl.ZedGraphControl_KeyDown( ) and ZedGraphControl.ZedGraphControl_KeyUp( ) from executing by returning true from their KeyDownEvent and KeyUpEvent event handlers.

[Tim Grudt]

ZedGraphControl modifications - based on 5.0.8