Gathering the set of files-to-watch

[David Nolden]

Ok, so I guess the right thing to watch is all the files you get in
ParsingEnvironmentFile::modificationRevisionSet, because that's
basically exactly what the language has added as "dependencies" to the
file.

Generally for performance reasons, you should also avoid to load all
the TopDUContext's that belong to the project, as that is slow.
Instead, you should better just request the ParsingEnvironmentFiles
directly through DUChain::environmentFileForDocument or
DUChain::allEnvironmentFiles.

Another important question is when add update the watches. I guess you
should add the watches once after starting, and then listen to
BackgroundParser::parseJobFinished to update the watches for the
specific file that was parsed.

Greetings, David