Qt: Unparent NetPlay dialog from main window

This should make the NetPlay dialog appear as a separate window in the taskbar on most systems, which makes more sense than a parented dialog as the user will leave it open for an extended period.

Source/Core/DolphinQt2/NetPlay/NetPlayDialog.h

@@ -30,7 +30,7 @@ class NetPlayDialog : public QDialog, public NetPlayUI
 {
   Q_OBJECT
 public:
-  explicit NetPlayDialog(QWidget* parent);
+  explicit NetPlayDialog(QWidget* parent = nullptr);
   ~NetPlayDialog();
 
   void show(std::string nickname, bool use_traversal);