Patch: Query favourites

This patch adds a favourites menu to the query tool, where one can store
often-used queries.

It brings along build dependencies on wxxml2
(http://wxcode.sourceforge.net/components/wxxml2/) and libxml2 (which
comes from the first). These dliver general XML support, which I'm sure
will be useful for a lot of other things as well - I'm already looking a
bit at some xml import/export functions that I'd like to integrate with
pgadmin, and that's just one thing. I think it'd be good in general to
have it.

The package builds easily with configure/make/make install on unix (at
one point I had to do a fromdos on the configure file, but I thin kthat
was because I dl:ed the wrong package). On windows, there are build
files for VS that worked fine in VS2003. Don't have VS6 around to test
it with.

Attached is one patch that is the actual functionality, and one against
the Unix build system. I'm no autoconf master by far, so that one may be
a bit off ;-) As said before, I don't have VS6 so I haven't got a patch
for that one. It should be a trivial matter of adding the files to the
project, though, and disabling PCH for the favourites.cpp file due to
version conflicts between wxxml and wxxml2.

Tested on win32, and a bit on linux (slackware on developer.pgadmin.org)
but not as much.

//Magnus