Adds OS cursor support

Hello,

Another minor pull request. This one intercepts the `WM_SETCURSOR` message on the `HtmlLabel` and `HtmlPanel` controls in order to override how the "hand" cursor is displayed and use the OS version rather than the .NET one when hovering over a link.

Note that the `HtmlToolTip` component will still use the "old" hand cursor, off the top of my head to make that one work I'll need to add a new class inheriting from `NativeWindow` to subclass the window you've already captured from `_tooltipHandle` but I'll save that for when I'm actually working with that component.

Sanity tested on XP and 8.1 just to make sure I hadn't forgotten anything.

Regards;
Richard Moss