Added HtmlPanel.BorderStyle

This pull requests adds a `BorderStyle` property to the `HtmlPanel` control to control the Win32 basic border style. I'm already aware that you can add CSS border styles to the `body` or `html` elements within the control, but if you want a true border around the whole control (including the scrollbars), then this is the best way.

I briefly considered adding the same to the `HtmlLabel` but I'll do that later on down the line assuming this particular one is accepted, and also I felt it was more useful on the `HtmlPanel`.

Let me know if these contributions are useful or if I should just keep them tucked in my own fork where they don't bother anyone ;)

Regards;
Richard Moss