This fix the wrong fill when fill="none". The original version fill it in black.

Support "none" in Fill value if inherited from parent

If elements are nested in groups and have different fill values than
their containing group the fill value wouldn't be written correctly.

- Changes to get texts displaying bold / normal weight
- Changes to get textspans work correctly
- Made (semi-hardcoded) document metadata section - allow reading and regurgitating metdata tag from an original source document
- Included image
- Modified paths to allow for scientific notation
- Included ability to have markers for path ends (arrow ends etc)

* added few enum converters

* fill can be null -> none in XML
* added type converters for enums
* added type converter for unitcollection
* fixes several viewbox bugs
* document can now draw into a given bitmap
* content of SVG tag is written (used by the text tag)
* changed font handling in text element, still need proper alignment
* changed intersparse character of SvgTransformConverter.cs to space, because inkscape couldn't parse comma
* added class diagram for basic shapes

Applies default values of 50% for radial gradient cx,cy and r. This prevents an OutOfMemory exception when loading svg files without these parameters specified.