imwayland: Don't reset serial while text-input is alive

dczrequested to merge
dcz/gtk:imwayland_serial into gtk-3-24

The serial number is a persistent property of the text-input object.

Hopefully fixes https://bugzilla.redhat.com/show_bug.cgi?id=1628462 and mutter#302 (closed)

Please test it out, I could only test with wlroots/some Geany.

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