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);

Isar_Document.reported_positions: exclude proof state output;

Document_View: less aggressive background coloring, departing from classic PG highlighting;
tuned colors;

report token range after inner parse error -- often provides important clues about misunderstanding concerning lexical phase;
tuned color;

slightly more robust Plugin.stop -- components might refer to Isabelle.system even after shutdown;

merged