Launcher: use QDesktopWidget to get display list instead of SDL2

First of all SDL2 trying to use dbus and this conflict with Qt. Check mantis#2704.
And there no reason to use SDL2 in launcher when Qt can do the same thing as well.

launcher/main.cpp

@@ -10,13 +10,11 @@
 #include <QApplication>
 #include "StdInc.h"
 #include "mainwindow_moc.h"
-#include "sdldisplayquery.h"
 
 int main(int argc, char * argv[])
 {
 	QApplication vcmilauncher(argc, argv);
-	auto displayList = getDisplays();
-	MainWindow mainWindow(displayList);
+	MainWindow mainWindow;
 	mainWindow.show();
 	return vcmilauncher.exec();
 }