Better check for when clipboard or primary selection have changed

Better check for when clipboard or primary selection have changed

Previously it was done by just comparing new and old selection
text, now we use also selection timestamps for systems that
support it (only enabled in X for now).  (bug#53894)

* lisp/select.el: (gui--last-selection-timestamp-clipboard)
(gui--last-selection-timestamp-primary): New variables.

(gui--set-last-clipboard-selection)
(gui--set-last-primary-selection)
(gui--clipboard-selection-unchanged-p)
(gui--primary-selection-unchanged-p): New functions.