bug#53894: 27.2; Clipboard copy -> C-y -> M-y -> Same clipboard copy does not push to kill ring

[Ignacio Casso]

> I also think that using symbol properties for
> gui--last-selected-text-clipboard/primary would be better, since a)
> using text properties implies some checks that the variable values are
> actually strings, and b) those text properties could be modified
> elsewhere, since we don't know with which other elisp code those string
> are shared. In particular, it could be the case that
> gui--last-selected-text-clipboard and gui--last-selected-text-primary
> shared the string (although if that was a problem it could easily be
> solved by using different timestamp property names for clipboard and
> primary). I've never worked with either text or symbol properties
> before, so I'm not sure. What do you think?

Another argument in favor of using symbol properties: the comparison to
not duplicate elements in the kill ring when the variable
kill-do-not-save-duplicates is non-nil is made with
equal-including-properties, according to the docstring of the variable.