New property 'context-menu-functions' (bug#62250)

New property 'context-menu-functions' (bug#62250)

* lisp/iimage.el (iimage-mode-buffer): Set context-menu-functions
text property to '(image-context-menu)'.

* lisp/image.el (image-context-menu): New function.
(put-image): Set context-menu-functions overlay property
to '(image-context-menu)'.
(insert-image, insert-sliced-image): Set context-menu-functions
text property to '(image-context-menu)'.

* lisp/mouse.el (context-menu-map): Use mouse-posn-property
'context-menu-functions' and call its funs at the end.