Improve output markup (pgml.xsl)

[David Aspinall <da+pgtrac@…>]

Output markup can and should be improved a lot by extending the XSL file for PGML.

• More x-symbol compatibility to support Isabelle symbol usage may be possible (e.g., superscripts/subscripts).
• Can we make some parameters which can be configured in the interface to set fonts/colours, etc?

Longer term it is frustrating to have the input/output disparity so great. The HTML view may be better replaced with a StyledText once that is flexible enough and a semi-wysiwyg approach is possible.