[Development] Use of std::function in Qt API

[Ulf Hermann]

> All that more or less already applies to the standard library however
> (probably most distros don't accept a standard library BC break without
> a mass rebuild anyway), so Qt insulating against BC breaks in the
> standard library is maybe less necessary.

This is the important observation. Hardly anyone (actually, is there anyone besides Qt?) guarantees resilience against standard library BC breakages. So, if the standard library breaks BC and the distribution accepts the new version, then it doesn't really matter what Qt does with it as all packages that use the standard library, including ones that also use Qt, will have to be rebuilt. C++ packages that don't use the standard library are rather rare.

Let's just allow standard library types in Qt, and document that the BC guarantee only extends across compatible standard libraries.

Ulf