citra_qt: Migrate to Qt 5 signal/slot connection syntax where applicable

This is more type-safe than the string-based signal/slot syntax that was
being used. It also makes the connections throughout the UI code consistent.

src/citra_qt/bootmanager.cpp

@@ -292,6 +292,6 @@ void GRenderWindow::showEvent(QShowEvent* event) {
     QWidget::showEvent(event);
 
     // windowHandle() is not initialized until the Window is shown, so we connect it here.
-    connect(this->windowHandle(), SIGNAL(screenChanged(QScreen*)), this,
-            SLOT(OnFramebufferSizeChanged()), Qt::UniqueConnection);
+    connect(windowHandle(), &QWindow::screenChanged, this, &GRenderWindow::OnFramebufferSizeChanged,
+            Qt::UniqueConnection);
 }