Document_View: more precise painting of gutter icons, only if line selection area is sufficiently large;

basic support for warning/error gutter icons;

Document_View: some markup for main inner syntax categories;

Command.State.accumulate: check actual source range;

Isar_Document.reported_positions: slightly more robust treatment of positions outside the command range, notably parsing beyond EOF;

highlight bad range of nested error (here from inner parsing);