QString::toAscii() doesn't exist in Qt5, using toLatin1() instead.
QImage::setNumColors() renamed to setColorCount()
QThread::finished() no longer emittable by subclasses.
QApplication::UnicodeUTF8 no longer exists as it is now the default.
qVariantValue no longer exists, .value<T>() replaces it.
QDesktopWidget include now required for MainWindow.cc
QDesktopWidget() doesn't work in Qt5, use QApplication::desktop() instead.
Plotting data now defaults to drawing a line with no symbols.
Also deleted invalid line drawing options.
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 #722 from Susurrus/qt5_fixes

Qt5 fixes
Merge pull request #724 from Susurrus/dataplot_ui2

Plotting data now defaults to drawing a line with no symbols.
Fixed unused param warnings
Fix warning in waypoint class
Fix linux release build
Merge pull request #728 from DonLakeFlyer/LinuxWarning

Fix linux release build warning
Merge pull request #726 from Susurrus/qwt_update3

Update Qwt to 6.1
Remove use of Qwt5.x types as they're now deprecated.
Fix for ScrollZoomer being updated for Qwt6.x.
Update Qwt6.x update for QGCXYPlot.cc as QwtDoubleRectno longer exists