extended text presentation (attribute + damager/repairer) to support underline styles
make foreground colour preference optional
support underline style config for Isabelle source viewer
pulled up AbstractMarkupScanner from IsabelleMarkupScanner allow scanners with different markup sets
avoid NPE
support ACTIVE syntax class colouring (used for action markups, e.g. "sendback" links) - these are rendered as hyperlinks in editor
support "sendback" links in prover output;
added infrastructure for other ACTIVE markups links to be added in the future
cleanup remnants of old sendback and Browser-based editor implementation
Merge branch 'sendback' into isabelle2013