Fix OS X wxWebView ignoring JavaScript window.open() calls

On OS X (tested on 10.10.5) calls to window.open() with a different target
than _self were simply ignored. No navigation and no EVT_WEBVIEW_NEWWINDOW as
expected.

Unfortunately WebKit does not call the decidePolicyForNewWindowAction delegate
method in these cases. A new delegate method createWebViewWithRequest has been
implemented to handle this case and send EVT_WEBVIEW_NEWWINDOW as in other
ports.

Closes https://github.com/wxWidgets/wxWidgets/pull/85

include/wx/osx/webview_webkit.h

@@ -145,6 +145,7 @@ public:
     void OnMouseEvents(wxMouseEvent &event);
 
     bool m_busy;
+    bool m_nextNavigationIsNewWindow;
 
 protected:
     virtual void DoSetPage(const wxString& html, const wxString& baseUrl);