Deprecate value"dash" for style:leader-width

The style:leader-width attribute has a value "dash". This value actually seems to be the result of a copy and paste error in the schema. It should either be deprecated, or entirely removed from the schema.

The same issue exists for style:text-underline-width, style:text-overline-width and style:text-line-through-width, which all share the same attribute value definition.