Add keyup to non windows drivers

[tig]

This PR adds support to OnKeyUp and OnKeyPress to the curses and net drivers (was only previously implemented in Windows). There is no concept of OnKeyDown for Curses and Net because the API is not rich enough.

This also fixes a bug in core.cs where OnKeyPress was acting on key down. It should act on key up, so that it can work consistently across all drivers.

The other thing this PR does is make it really easy to process key strokes for any Window (where previously one had to subclass the Window and provide an implementation of ProcessKey):

		Win.OnKeyPress += (KeyEvent ke) => {
			if (ke.Key == Key.Esc) {
				RequestStop ();
			}
		}; 

[tig]

The latest update to this PR fixes:

• The regression @BDisp reported where key-repeat broke
• KeyEvent (Event.cs) is a struct, but has a constructor; change to class and initialize Key to Unknown #444
• Bugs in WindowsDriver and CursesDriver that caused some KeyUp events to show ControlSpace instead of Unknown.

This PR is needed for UI Catalog to function properly.

[BDisp]

In Windows the repeat issue continues. It only write the key after I release it. Well, the KeyPress must be invoked after a valid key is present. If I press 'a' it should be written even I don't release the key. On Curses is working as espected.

[tig]

This PR is not needed. Already merged.