Fix #618 / Subobject subquery

- Relates to #618, #454
- Requires #626
- Add `SMWQueryParser::isPagePropertyType`
- Add test for  `[[Has subobject::<q>[[Area::891.85 km²]] OR [[Population::2234105||3517424]]</q>]]` or `[[Berlin]] [[Has subobject::<q>[[Area::891.85 km²]] [[Population::3517424]]</q>]]`.