qt: Flush output after write for QProcess output

* lang/qt/src/dataprovider.cpp (QIODeviceDataProvider::write): Call
waitForBytesWritten.

--
The problem here is that QProcess writes into an internal buffer
which is written to stdin of the process triggered by a
signal/slot connection. That connection is broken when we move
the QProcess into our GPGME thread and only restablished when
our Job is finished. This caused Kleopatra to basically keep
everything when decrypting a large archive in memory and
only write it out to the unpack process once the decryption
was finished.

GnuPG-Bug-Id: T5475

lang/qt/src/dataprovider.cpp

@@ -248,7 +248,17 @@ ssize_t QIODeviceDataProvider::write(const void *buffer, size_t bufSize)
         return -1;
     }
 
-    return mIO->write(static_cast<const char *>(buffer), bufSize);
+    ssize_t ret = mIO->write(static_cast<const char *>(buffer), bufSize);
+    if (mHaveQProcess) {
+        /* XXX: With at least Qt 5.12 we have the problem that the acutal write
+         * would be triggered by an event / slot. So as we have moved the io
+         * device to our thread this is never triggered until the job is finished
+         * calling waitForBytesWritten internally triggers a _q_canWrite which will
+         * actually write. This is what we want as we want to stream and not to
+         * buffer endlessly. */
+        qobject_cast<QProcess *>(mIO.get())->waitForBytesWritten(0);
+    }
+    return ret;
 }
 
 off_t QIODeviceDataProvider::seek(off_t offset, int whence)