Only warn about missing WebKit extension when used

Don't log failure to load the extension during initialization, because
it isn't needed for many uses of wxWebView. Instead, only report the
failure at the time when functionality depending on it is used.

Also use g_warning() for logging consistently with other failures in
this file. This doesn't interrupt the user, yet shows the problem highly
visibly in the console.

include/wx/gtk/webview_webkit.h

@@ -162,6 +162,7 @@ private:
 #if wxUSE_WEBVIEW_WEBKIT2
     bool CanExecuteEditingCommand(const gchar* command) const;
     void SetupWebExtensionServer();
+    GDBusProxy *GetExtensionProxy() const;
     bool RunScriptSync(const wxString& javascript, wxString* output = NULL);
 #endif