consistency in clue types

[Edd Dumbill <edd usefulinc com>]

OK, I've noticed some oddness going on with the current Epiphany and
Evolution frontends.

While docs/cluetypes.txt only notes 'textblock', I get different things
from these two frontends.

Evo sends only 'textblock'.  Unfortunately, it always sends HTML with
the entities escaped.  This seems to confuse dashboard.

Epiphany sends 'content' and 'title', which aren't documented.

There are two things we're missing which I think we need for textblocks:
content-type and charset encoding.  The way to get round charset is to
mandate that everything comes in in utf-8, which doesn't seem so
difficult.

Content-type is still reasonably important though, as we minimum need to
differentiate between text/plain and text/html.

I think the 'title' clue is probably quite useful: it's a much heavier
hint to possible relevance than just a textblock, but on the other handy
maybe that's what the relevance attribute is for (has anyone set this to
anything other than 10 yet? will people ever?).

-- Edd