Make qsizetype user-configurable

I'd like to have qsizetype user-configurable so that users with a 'int' based code base (such as Qt Creator) do not have to invest heavily into porting their code for no practical benefit.

I don't find written references to the original discussion on the introduction of qsizetype anywhere but unless my memory plays tricks this idea was discussed and part of the deal back then.

Simply changing the type alias doesn't work in current Qt 6 anymore as there are some hard asserts on sizeof(qsizetype) == sizeof(std::size_t) now, but from a quick look none of them seems absolutely necessary.

I'd also like to note that this request is /not/ incompatible with the (not necessarily supported by me, but known to exist) wish to have sizeof(std::size_t)-sized sizes for the string views to allow them to be used with std:: containers