Move quitting flag to manager quit

The flag is to protect against the frame going out of scope with the
references intact.  This shouldn't be in CloseProject because we close
the project without removing the frame.  Instead, placing it in OnExit
gets set correctly when closing the whole project manager

Fixes https://gitlab.com/kicad/code/kicad/issues/10018

kicad/kicad_manager_frame.cpp

@@ -394,6 +394,7 @@ void KICAD_MANAGER_FRAME::doCloseWindow()
 #endif
 
     m_leftWin->Show( false );
+    Pgm().m_Quitting = true;
 
     Destroy();
 
@@ -435,8 +436,6 @@ bool KICAD_MANAGER_FRAME::CloseProject( bool aSave )
 
     m_leftWin->EmptyTreePrj();
 
-    Pgm().m_Quitting = true;
-
     return true;
 }