Enable HiDPI Scaling (Qt 5.6+ only) [stable-1.2] (#3814)

Enables HiDPI Scaling in GuiApplication.cpp, environmental variable and mac manifest

src/gui/GuiApplication.cpp

@@ -40,6 +40,7 @@
 #include "SongEditor.h"
 
 #include <QApplication>
+#include <QtGlobal>
 #include <QMessageBox>
 #include <QSplashScreen>
 
@@ -53,6 +54,11 @@ GuiApplication* GuiApplication::instance()
 
 GuiApplication::GuiApplication()
 {
+	// enable HiDPI scaling before showing anything (Qt 5.6+ only)
+	#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0))
+		QApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+	#endif
+	
 	// prompt the user to create the LMMS working directory (e.g. ~/lmms) if it doesn't exist
 	if ( !ConfigManager::inst()->hasWorkingDir() &&
 		QMessageBox::question( NULL,