Fixes to botched Qwt 6.1 update

Update Qwt to 6.1

Fix linux release build warning

Plotting data now defaults to drawing a line with no symbols.

Qt5 fixes

I was unsure how to update the QwtPlotPrintFilter code, so I just commented it out with a FIXME.

We also document how we got the source code and keep our modifications out of the Qwt directory.

Also deleted invalid line drawing options.

espeak: provide string length including 0 character

Removed mavlinkgen application.