qt: Add setting to prompt for user on game boot

Using the QtProfileSelectorDialog, this implementation is trivial. This mimics the real switch behavior of asking which user on every game boot, but it is default disabled as that might get inconvenient.

src/yuzu/main.h

@@ -128,6 +128,8 @@ private:
     void ShowTelemetryCallout();
     void SetDiscordEnabled(bool state);
 
+    void SelectAndSetCurrentUser();
+
     /**
      * Stores the filename in the recently loaded files list.
      * The new filename is stored at the beginning of the recently loaded files list.