Projects now contain a set of TextDocument objects. Each TextDocument
represents a member file in the project. TextDocuments may not have
their file contents loaded at all times, but they will be loaded on
demand when calling TextDocument::contents().
"Find in files" works by iterating over the documents in the project
and calling find(needle) on each one. The return value from find() is
a vector of line numbers where the needle was found.
This is obviously going to need a bunch more work. :^)
