SPCSSAttr should be merged with SPStyle

[Tavmjong Bah]

The class SPCSSAttr (actually Inkscape::XML::Node)and supporting functions should be merged with SPStyle as they duplicate most of the same functionality. SPCSSAttr is used for temporary storage of a list of properties parsed from the 'style' attribute. Properties are then added or deleted from the list before writing the properties back to the 'style' attribute.

While SPCSSAttr is derived from Inkscape::XML::Node it doesn't appear to use any of the special features of Inkscape::XML::Node. It could have been implemented via a std::list of strings.