[referencer] lyx plugin

[John Spray]

I've found that I can use the lyxclient tool that comes with lyx to
insert citations without altering my lyx configuration.  I believe it
uses sockets rather than pipes.  I've hacked the lyx plugin to use
lyxclient instead of pipes (sorry for demolishing your code).  Using an
external tool is not ideal, but I think it's important to "Just work"
rather than needing to provide instructions on altering lyx config.  I
believe lyxclient is part of a default lyx install, at least on linux
(which is currently the only place referencer runs).

I've committed your code for setting icons, thanks.  It is a bit
convoluted but I think that's more GTK's fault than yours.  Anyway, it
works and it's self-contained, so I don't think it's a problem.  I've
added data/lyx.png.

Perhaps the python code that the lyx people have would let us do this
without using external program, in which case that would be a neater
way.

I've taken the random print statements out of lyx.py, although you'll
find that the between-release svn code is often thick with overly
verbose stuff like that.  That one was a little gratuitous, I agree.

I think we're pretty much good to go, apart from a bit more error
handling stuff that I'll do on the C++ side.

John