Fix Linux HiDPI handling

Fix HiDPI order of operations, remove shortcut technique

cmake/linux/lmms.desktop

@@ -8,7 +8,7 @@ Comment=Music sequencer and synthesizer
 Comment[ca]=Producció fàcil de música per a tothom!
 Comment[fr]=Production facile de musique pour tout le monde !
 Icon=lmms
-Exec=env QT_X11_NO_NATIVE_MENUBAR=1 QT_AUTO_SCREEN_SCALE_FACTOR=1 lmms %f
+Exec=env QT_X11_NO_NATIVE_MENUBAR=1 lmms %f
 Terminal=false
 Type=Application
 Categories=Qt;AudioVideo;Audio;Midi;

src/core/main.cpp

@@ -252,7 +252,9 @@ int main( int argc, char * * argv )
 		return EXIT_FAILURE;
 	}	
 #endif
-
+#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
+	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
 	QCoreApplication * app = coreOnly ?
 			new QCoreApplication( argc, argv ) :
 					new MainApplication( argc, argv );

src/gui/GuiApplication.cpp

@@ -54,11 +54,6 @@ 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. ~/Documents/lmms) if it doesn't exist
 	if ( !ConfigManager::inst()->hasWorkingDir() &&
 		QMessageBox::question( NULL,