qt: Log messages with type>QtDebugMsg as non-debug

More important messages should end up in the log no matter
if -debug=qt is set.
qt: Change serious messages from qDebug to qWarning

By changing the logging stream for warnings from qDebug to qWarning,
these will always be logged to debug.log.
Merge pull request #4448 from laanwj/2014_07_qt_payment_rq_logging

qt: Always log warning/error messages to debug.log