On the toplevel of an image description you can now
specify include directive(s) like in the following
example
<image ...>
...
<include from="filename_a.xml"/>
...
<include from="filename_b.xml"/>
</image>
At the place of their occurrence the include statement
will be replaced with the contents of the given filename.
The implementation is based on a XSLT stylesheet and
applies very early in the process. The stylesheet reads
the contents of the given file as XML document().
Thus only valid XML documents gets accepted by this
include concept. This Fixes#1929 and is related to
Issue #1918
ef05ecc36e