Moved settings for concordance building into Hits.

This allows us to change the concordance type we want per query instead
of per Searcher.

Also introduced get/setConcordanceType() to replace the "use forward
index to make concordances" boolean setting, and fixed concordances from
the content store not being guaranteed to be well-formed.
Fixed DocResult snippets not copying settings from source Hits object.
Added "remove" strategy for fixing well-formedness of snippets.

Snippets and concordances might not be well-formed XML because they are
cut from a larger XML document. To fix this, we used to always add tags
to the start or end of the snippet. This might produce strange results
(i.e. sentence start tags mid-sentence). The new "remove" strategy might
be a better option in this case.

Theoretically, we could enhance the system in the future by providing
per-tag strategies, but this seems overkill for now.
Added DocContents classes and used them in Kwic and Concordance.

DocContents is an abstract base class with two child classes,
DocContentsString and DocContentsFromForwardIndex. They represent a part
of the document contents, either from the original input data or
reconstructed from the forward index.

Kwic and Concordance now use these to store the part of the content they
work with. We also plan to use them to implement generic "get part of
the document content" methods.
Made fetching and highlighting parts of the document possible.