[Development] QtSingleApplication in Qt5

[Joseph Crowell]

I got the old Qt4 QtSingleApplication addon compiling under Qt5 by 
updating the includes and removing the QT_WA macro. But it doesn't seem 
to be detecting the application any more under Windows. Could anyone 
take a guess on why it isn't detecting open instances any longer? I also 
changed Q_WS_X11 to Q_OS_LINUX and Q_WS_WIN to Q_OS_WIN in the places 
where they were used.