iOS: Memory leak in QPlatformClipboard

There are two memory leaks in the mime data handling in QClipboard on iOS.

The first is in QIOSPlatformClipboard::mimeData(). This is expected to return a pointer to internal memory which is managed by the clipboard, but on iOS it just returns new objects every time and these are never deleted.

The second is in QIOPlatformClipbard::setMimeData(). This is documented to take ownership of the QMimeData object, but on iOS it only fetches the data from the object and returns without deleting it.