New thing-at-point target 'list-or-string' used in context-menu-region

New thing-at-point target 'list-or-string' used in context-menu-region

* lisp/mouse.el (context-menu-region): Rearrange the order to
All>Defun>List>Line>Symbol.  Show title either "List" or "String"
depending on syntax-ppss, then use thing 'list-or-string' (bug#9054).

* lisp/thingatpt.el (thing-at-point-bounds-of-list-at-point): Add optional
args 'escape-strings' and 'no-syntax-crossing' like in 'up-list'.
(list-or-string): New target 'list-or-string'.
(thing-at-point-bounds-of-list-or-string-at-point): New function.