misc tuning and simplification;

renamed Markup_Tree.Node to Text.Info;
Markup_Tree.select: body may depend on full Text.Info, including range;
tuned;

removed obsolete Markup_Tree.flatten/filter;

Isabelle_Hyperlinks: select relevant information directly from Markup_Tree, without intermediate RefInfo;

Document_View.text_area_extension: select relevant information directly from Markup_Tree, without intermediate TypeInfo;

tuned signatures;

Command.State: accumulate markup reports uniformly;
Document_Model.token_marker: select relevant information directly from Markup_Tree, without intermediate HighlightInfo;
tuned;

Markup_Tree.select: misc simplification, proper restriction of parent in subtree;

added Text.Range.- convenience;

tuned signatures;