very simple datastore reimplementation

[Yoshiki Ohshima]

> and that's it. It's a trivial thing, will work on any fs, on any OS,
> no magic tricks needed. We can do fast searches on based on the
> "documents metadata", and the only "slow" op is mounting a device
> where the documents metadata is stale or missing.

  I like along this line, too.  In fact, Journal can be mostly a
smarter version of "recent documents" feature.  That can hide the
hierarachical file system, and present "documents" in the time-based
view.

  On a file system that doesn't support the owner/group permission, an
activity could mess up the other activities data; but the user can do
it from the command line even for the internal file system more or
less.

  I would imagine that separated metadata can be optional.  For
Text-type documents (including Etoys), we can as well just make the
tags in the document itself.  For a picture, it can be EXIF and can
have comments, etc.  Any file can have comments, and since we are
operating some limited kind of files anyway, the Journal can support
them one by one.

  On a related note: tagging the document at a separate place wouldn't
work well (http://dev.laptop.org/ticket/6128).  This is another reason
to make the "tags" just appear inside documents.

-- Yoshiki