gimp/devel-docs/tools

The doc-shooter is a small utility used to create screenshots of
widgets for the GIMP API reference manuals.  This utility is X11
specific, so it should only be built when GIMP is being compiled
for the X11 GDK backend.

Taking the screenshots is not part of the docs build.
Instead, the images are created, and then copied by hand into
devel-docs/libgimpwidgets/images.  Ideally, the images should be
taken once a release, and all images should be updated at the
same time. A simple theme should be used to take the screenshots.

The GTK+ docs team came up with the idea for this utility as
well as some of the code to implement it.