Tweaked closing behaviour (BohemianCoding/Sketch#3232).
By default now a new popover will close an existing one.
There are now two delegate methods that give both the new and the old popover the chance to customise this if necessary. Sketch doesn't actually need to implement either of them currently.
Merge pull request #1 from BohemianCoding/feature/3232-delay-closing-popovers

Delayed closing.