Refactor to allow more converter objects, investigate conversion mechanisms

[David Aspinall]

PGIPIncoming has a converter member for converting text from the prover to a displayable form. Each session manager makes one converter object which is set in the incoming events, but unfortunately converters are single-threaded so become a bottleneck if we want to convert several text objects at once. To address this, we should refactor to include a Prover field instead of a converter, and then we could use a small pool of converter objects generated by a factory method in Converter. (Note: they shouldn't be generated freshly for each conversion as it requires compiling XSLT!).

Additionally we could look at other conversion mechanisms, e.g., maybe CSS mechanisms instead could improve performance.