Allow combining custom XSLT with theme generated XSLT

[jelovirt]

Support combining custom XSLT with theme generated XSLT.

[jelovirt]

It's a feature that clearly some users would find helpful, but the development experience would be poor. Part of the code is generated, so it's not available when writing the custom XSLT. You can work around this by saving the generated XSLT from the temporary directory and use that when developing the custom XSLT. However, when the a new version of the PDF2 Theme plug-in is released, the generated XSLT can change drastically even if your theme file is still the same. This might cause incompatibilities between the generated and custom XSLT.