HTML 5 support in the WYSIWYG editor

The WYSIWYG Editor Module now uses HTML 5. See below for additional notes on backwards-compatibility. Not all HTML 5 features are supported, yet, the main new feature is support for parsing figures with captions. There should be a closer match between the HTML code in the editor and the HTML code in view mode but this might require adaptations for WYSIWYG editors and extensions extending them. This has also consequences for renderers that should handle figure and figure caption events not just in macro contexts (due to the figure macro).