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

[Ignacio Casso]

> My worry after skimming this is that you are making X-specific changes
> in places that are used by non-X GUI environments and window-systems.
> Can't we do all that entirely in X-specific code, so that the
> probability of breaking other window-systems would be strictly zero?

I've assumed that for GUI environments where (gui-get-selection
'CLIPBOARD 'TIMESTAMP') would not return an actual timestamp as
intended, it would return nil. Under that assumption (which I don't
really know if is correct), the new semantics for those systems is
exactly the same as before. But I can try to move the logic to x-win.el
or check the window-system variable when necessary. I will implement Po
Lu's suggestion of storing the timestamp as a text property first, and
then we can revisit this.