bugfix/gdk-pixbuf (#10312)

* docbook processing works correctly for gdk-pixbuf

1. The various bits of documentation in gdk-pixbuf include hardcoded
   references to dtd's and stuff at their canonical, Internet,
   locations.  BUT, gdk-pixbuf runs xslt-proc with the `--nonet`
   option, which forbids it from using the network.  Sadness ensues.

   Traditionally folks use XML Catalogs to map these to local copies.
   Our docbook-xsl package wasn't setting the appropriate env var in
   its dependents environments to use our catalog.

   Now it does.  Less sadness ensues.

2. If we're going to use these things, we should depend on them.

* Add gdk-pixbuf's bin to build environment

The "post-install.sh" script uses gdk-pixbuf-query-loaders, which was
installed earlier.

If py-psyclone can set its own bin on its PATH, so can we...

* Make gdk-pixbuf use recognizable docbook location

Our docbook-xsl package assumes that the canonical location is
`http://cdn.docbook.org/release/xsl/current/manpages/docbook.xsl`, but
the gdk-pixbuf's meson build script uses
`http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl`.

This means that our XML Catalog doesn't fix the reference and sadness
happens.

Just patch the build that we see what we want to see, then we can make
it all go away.

* Add commentary re docbook patch

* Make catalog helper a property

Thanks @adamjstewart!

* Run tests if/when run_tests is true

Thanks @adamjstewart!

* Tune up dependencies

Thanks @adamjstewart!

* Wordsmit commentary

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