KeyDown and KeyUp event support

[tig]

For an enhancement to MenuBar I'm working on I need access to alt key down events.

I discussed it a bit here: #366

This pull request delivers that in a generic/flexible way.

The essence is a set of new virtual members on Responder:

public virtual bool KeyDown (KeyEvent keyEvent)
public virtual bool KeyUp (KeyEvent keyEvent)

Then on View a set of overrides of those virtuals plus convenient Actions:

public Action<KeyEvent> OnKeyDown;
public Action<KeyEvent> OnKeyDown;

Users can then either implement KeyDown or KeyUp in derived classes, or use the Actions for objects they hold:

win.OnKeyDown += (KeyEvent keyEvent) => ...

demo.cs has been updated with a simple demo.

Caveat: It appears that only the WindowsDriver is actually capable of capturing and sending distinct key up and key down events. I studied both CursesDriver and NetDriver and couldn't find any evidence that they are capable of sourcing these events. I would love to be proven wrong. Regardless, the design of this PR means that apps that choose to utilize these events will need to do so understanding that the won't get them anywhere but on Windows.

[migueldeicaza]

I think this looks good, will merge soon. Will have to do by hand as there are a lot of white space changes

[tig]

Looks like you took care of this already.