Add a function to QFile that creates a file including any parent directories

QFile::open() with WriteOnly will create the file if it doesn't exist, but it won't create parent directories. QDir::mkpath() only works with directories.

For example, it would be nice to be able to do this:

QFile file("/tmp/this/will/be/created.txt");
if (!file.createAndOpen(QIODevice::WriteOnly))
    // ...

Or maybe add an overload of open that takes an enum to control the directory-creating behaviour:

enum CreationPolicy {
    DoNotCreate,
    CreateFile,
    CreateParentDirectoriesAndFile
}