Reduce the memory footprint of rendered ontology views.

Avoid keeping rendered text around when it isn't being displayed. 

Use custom writer, content, and document classes to avoid having to keep up to three copies of the 
rendered text. 

Render text in a separate thread, displaying a progress count until rendering is finished.

Contains a workaround for an exception masking issue in  the version 3 owlapi manchester syntax object renderer (the fix was back ported a couple of hours ago).