Add access to user agent for wxWebView

Allow setting a custom user agent for a webview.
Also allow access to the current user agent.

include/wx/webview.h

@@ -187,6 +187,8 @@ public:
     virtual void Print() = 0;
     virtual void RegisterHandler(wxSharedPtr<wxWebViewHandler> handler) = 0;
     virtual void Reload(wxWebViewReloadFlags flags = wxWEBVIEW_RELOAD_DEFAULT) = 0;
+    virtual bool SetUserAgent(const wxString& userAgent) { wxUnusedVar(userAgent); return false; }
+    virtual wxString GetUserAgent() const;
 
     // Script
     virtual bool RunScript(const wxString& javascript, wxString* output = NULL) const = 0;