svggen: customizable serialization in SVGGraphics2D

[carlosame]

Some people have complained that the SVG output by Batik's SVGGraphics2D is ugly or too big, see BATIK-502 or BATIK-1337. The serializer that svggen used was package-visible because it wasn't generically valid, and there was no way to customize the serialization.

This PR offers:

• A public serializer which has general validity.
• A new XmlSerializer interface that can be used to customize the serialization. Its default methods mimic what the previous code did.
• A CompactXmlSerializer implementation which significantly decreases the size of the output files.
• Methods for setting and getting a custom serializer to SVGGeneratorContext. The default is backwards-compatible as mentioned above.

To use the compact serializer just set it:

SVGGeneratorContext context = ...
context.setXmlSerializer(new CompactXmlSerializer());