Add 'osb' to list of namespaces ignored when checking attributes/properties.

Add JessyInk namespace.

Protect against missing attributes and property files.

Add possibility to check validity of attributes and usefulness of properties. This code adds the ability to check for every elment in an SVG document if its attributes are valid and the styling properties are useful. Options under the SVG Output section of the Inkscape Preferences dialog control what should be checked when, and what actions should be taken if invalid attributes or non-useful properties are found.