Fix nested QVariantLists in m_outputBuffer/readData()

In processCombinedData(), we create a QVariantList, to which we append the data from m_inputBuffer, and output buffers of any input funnels, and set this to be the value of m_outputBuffer. However, since all the m_outputBuffers of the input funnels will also be QVariantLists (even if it just contains one QVariant), we end up with nested QVariantLists if the funnel has any input funnels set.

If we want the output buffer to return a QVariantList, that is fine, but it should IMHO always be just one QVariantList, and the output of the inputs should be a single QVariant in the list, at least in cases where the list only contains one QVariant. Also, if we intend the output buffer to always return a QVariantList, would make sense to explicitly set the return type to that to avoid surprises.