Sparse indexing

[teodosin]

There's no need to index all items always in a directory. The constraint that Karta can only show nodes in its own database is arbitrary. It should load files, directories as nodes, and only if some karta-only data is set on them, such ass pins, attributes, or connections, should they be indexed in the database. Such sparse indexing will be very important for large directories with hundreds or thousands of files, so the db doesn't get immediately bloated. Also in the future, when file destructuring gets implemented, it will be very useful to link just parts of a file to the wider graph without having to index the whole thing. If all the node data is already implicit in the file or directory, the node wrapper isn't needed.

For the current impl this just means that opening a context should also open the physical files without them having to be indexed. Take care to check for already indexed nodes.