clarified bootstrap of @{make_string} -- avoid query on ML environment;

Pure attribute setup is back to Pure/Isar/attrib.ML, where it can be editing continuously (see also 7eb0c04e4c40);

prefer regular context update, to allow continuous editing of Pure;

clarified editor mode;

treat ROOT.ML as theory with header "theory ML_Root imports ML_Bootstrap begin";

more robust bootstrap;

virtual thread data via context, for proper support of Context.>> etc;

unused;

tuned signature;

clarified bootstrap;