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/debugger/graphics/graphics.cpp

@@ -10,7 +10,8 @@ extern GraphicsDebugger g_debugger;
 
 GPUCommandStreamItemModel::GPUCommandStreamItemModel(QObject* parent)
     : QAbstractListModel(parent), command_count(0) {
-    connect(this, SIGNAL(GXCommandFinished(int)), this, SLOT(OnGXCommandFinishedInternal(int)));
+    connect(this, &GPUCommandStreamItemModel::GXCommandFinished, this,
+            &GPUCommandStreamItemModel::OnGXCommandFinishedInternal);
 }
 
 int GPUCommandStreamItemModel::rowCount(const QModelIndex& parent) const {