Make QSvgRenderer internals more accessible

Since QSvgRenderer must parse the SVG, it would be useful to provide a more complete API for querying the SVG DOM. As it is, programs can query the existence of elements by id, get their bounding box, the document viewbox, and little more. Beyond that, a separate XML parser must be used to get information which MUST be readily available within QSvgRenderer, such as:

• a list of element IDs
• the document stylesheet
• element/s at co-ordinate
• element classes

And DOM manipulations, such as:

• Excluding elements from a render (visibility)
• Modifying stylesheets
• Modifying attributes