Flush in `File::write` when in unbuffered "text" mode

If the wrapped `QFile` was opened with `QIODevice::Unbuffered`, any
writes should be unbuffered.  However, as currently implemented,
using `QTextStream` when the `File` is in "text" mode causes all
reads/writes to be buffered.

This modification forces a flush in `File::write` if the wrapped
`QFile` was opened with `QIODevice::Unbuffered`.