Update Qwt to 6.1.
We also document how we got the source code and keep our modifications out of the Qwt directory.
Now update QGC to use the new Qwt 6.1 code.
I was unsure how to update the QwtPlotPrintFilter code, so I just commented it out with a FIXME.
Merge pull request #726 from Susurrus/qwt_update3

Update Qwt to 6.1