QFile: add support for copy() or rename() to overwrite an existing file

QFile::copy() does not copy the file if the target file aready exists. However, it would be nice if copy() had a boolean argument to indicate that the target file should be overwritten if it already exists, or if a new function was added to QFile to allow overwriting existing files.

This would for example make it more convenient to create a QTemporaryFile and copy the contents of an existing file into it.