merged markup backend

[Havoc Pennington <hp redhat com>]

Hi,

I added the markup backend to HEAD, that I had on a branch from quite
a while ago. It is not used by default so is mostly just sitting
there. It does pass all the test suite stuff so might work if you
changed your gconf path file to use it.

The main point of this backend is for the backend code to be halfway
decipherable, while the old xml- backend isn't. i.e. the main point is
to rewrite the backend because I do not understand the XML backend
code, and I'm pretty sure no one else does either. The use of gmarkup
instead of libxml is just a side effect I figured I'd do while I was
at it.

The markup and xml backends can each parse files generated by the
other.

Since the xml backend has no known bugs, and the markup backend
doesn't appear to be faster or otherwise better, I'm not sure I'm
going to be motivated to swap the default. Though if I was going to
add features to the backend, or did find xml backend bugs, I'd be
inclined to.  If you try to figure out the code of each one you'll see
why.

One thing I haven't checked is the relative memory usage of each
backend; they may differ in that respect, I don't know.

Havoc