Review request for wip-manual branch

[Neil Jerram]

The wip-manual branch contains various bits of work on the manual.
It's largely driven by the plan to publish the manual as a book, later
this year, and the biggest change so far is the merging of the GOOPS
manual.  Apart from that I'm not following any kind of systematic
plan, just reviewing and working on areas as I notice them.

One general thing is that I am consciously _not_ trying to make the
manual more consistent in style - i.e. closer to my own style, or to a
more `neutral' style, or whatever; I actually think it's quite fun
that the style and tone change a bit from place to place.  So my
overall guideline is only to change something when there is a clear
benefit from doing so, not just because I'd write it a bit
differently.

So, please let me know of any comments.  Subject to those, I'll
merge what's there already after the 1.9.2 release, and then keep
working on this as time permits.

Regards,
        Neil