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.
Merge pull request #722 from Susurrus/qt5_fixes

Qt5 fixes