Defer launch warnings until after launch validation

We need to destroy the SDL window before we can be sure that Qt
can draw to the screen if we used KMSDRM.

app/streaming/session.h

@@ -260,6 +260,7 @@ private:
     SdlInputHandler* m_InputHandler;
     int m_MouseEmulationRefCount;
     int m_FlushingWindowEventsRef;
+    QList<QString> m_LaunchWarnings;
 
     bool m_AsyncConnectionSuccess;
     int m_PortTestResults;