Annotation search: don't try to highlight without skeleton ID
Annotations: avoid unneeded conditional
Annotation search: add support for sub-annotation search

This commit adds a new checkbox to the right of each annotation input field in
the search. It is named 'Include sub-annotations'. If checked, the search
result will not only contain entities directly annotated with the requested
annotation, but also transitively annotated ones. Listing these was possible
before by expanding annotation entities. However, this option will include those
results already on the first level.