Many improvements for Context Menus (bug#9054)

Many improvements for Context Menus (bug#9054)

* lisp/menu-bar.el (menu-bar-showhide-menu): Add "Context Menus".

* lisp/mouse.el (context-menu-undo): Add "in Region" to the titles
when the region is active.
(context-menu-region): Use 'mouse-yank-from-menu' in menu items
created from 'yank-menu' for submenu "Paste from Kill Menu".
(context-menu-region): Add submenu "Select" with things to mark.
(mark-thing-at-mouse, mouse-yank-from-menu): New functions.

* lisp/thingatpt.el (bounds-of-thing-at-mouse): New function.

* lisp/progmodes/elisp-mode.el (elisp-context-menu):
* lisp/progmodes/prog-mode.el (prog-context-menu):
Use full symbol/identifier names in :help strings.

Suggested by Martin Rudalics <rudalics@gmx.at>