gdk/wayland: Use serial of the latest implicit grab available

gdk/wayland: Use serial of the latest implicit grab available

When getting the serial for primary/clipboard selections we used a
function that largely relied on a GdkEvent being passed. We have
another available function that looks up the most recent serial
given the ongoing touch/tablet input as well.

This is the second best, compared to actually knowing the
input/device from the event that was received by the UI an triggered
the clipboard operation, and is already in use in other places
(e.g. window dragging). It is valid for these situations too.

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5250