Mac OSX WebView - favicons, new windows, user agent

[garrybodsworth]

I've made a few additional changes whilst experimenting some more on the Mac.

• Favicons - when the webview gets a new favicon we base64 encode a PNG (suitable for a data: url).
• Inherit the user agent from the parent browser (using the relevant NPAPI interfaces).
• New window handling - proved to be a little tricky but new windows can now be captured with a URL (unfortunately this does not cover the case of parent<-> child relationships, but for URLs it will work).