QDirIterator → QDirListing should have some porting documentation

QDirListing is the new, recommended class, and we we want users to move to it. However, it's not a replacement that can be done completely automatic:

• The flags are different. Not only is the type different, but also the default behaviour when it comes to hidden files. That should be spelled out.
• While it's still possible to call it->fileInfo.foo(), it is now feasible to directly call it->foo() in more cases, and that should be recommended
• maybe something else?