Qt6: port from QDesktopWidget to QScreen
This is a bit of a messy commit since half-way through I moved the porting utility functions from kis_global.h to a new header, KisPortingUtils.h, which also affects all the places where we used QTextStream.
This is a bit of a messy commit since half-way through I moved the porting utility functions from kis_global.h to a new header, KisPortingUtils.h, which also affects all the places where we used QTextStream.