plugins/slidingpopups: use standard duration values
Right now the opening duration is 150ms, and the closing duration is 250ms. Neither are standard durations we use elsewhere, and it's also a bit odd to be slower to close than to open--at least from a visual perspective.
The average of these value happens to be 200ms, which happily is the exact value of Kirigami.Units.longDuration.
As such, use that value instead, in the interests of visual consistency.
No screen recording because the difference is almost imperceptible to me, so hopefully this won't be controversial. :)