Added parameter to the SvgAttribute indicating whether the storage is
the _attribute dictionary.  If true, the output will only be serialized
only if the key is in the dictionary.  This structure is an alternate to
the SvgDefinitionDefaults class.

- 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)

- Adding initial support for Write functionality. Units & Transforms mostly. Enums still need work. Needs support for an IsDirty concept so that that every possible attribute isn't written.

- Added SvgMatrix, SvgShear and SvgSkew missing files from the previous check-in.