Deprecated methods so DocResults, DocGroup, etc. can become immutable.

add() methods were deprecated in preparation for a future update that
will make these classes immutable, which simplifies writing correct
multithreaded applications such as BlackLab Server.
Fixed some Hits constructors not counting documents.

The Hits constructors that take a List<Hit> didn't count the number of
hits.
Fixed Searcher.queryDocuments() not always working correctly.

Some filter queries would not produce any hits at all. This was likely
to do with using Scorer. Now we use a Collector, guaranteeing we get all
matching documents.
Added basic support for docs-only queries in QueryTool.

To use this, specify a filter (e.g. "filter title:dog") and type "docs"
to search for matching documents. Alternatively, search for hits first,
then type "docs" to switch to docs view. Also added a "clear" command to
clear the filter and results.