Implement dismissal for unfocused wxPopupTransientWindow

Popups not using wxPU_CONTAINS_CONTROLS were not automatically dismissed
at all any longer, as the only auto-dismissal mechanism related on
getting WM_ACTIVATE, which they never did, so implement a different
logic for dismissing them: do it on any change of focus and also any
mouse press (but not move and not key press neither).

This will allow not using wxPU_CONTAINS_CONTROLS for popups that don't
need focus, but still must disappear on their own.

include/wx/msw/popupwin.h

@@ -40,6 +40,10 @@ public:
     // Return the top level window parent of this popup or null.
     wxWindow* MSWGetOwner() const { return m_owner; }
 
+    // This is a way to notify non-wxPU_CONTAINS_CONTROLS windows about the
+    // events that should result in their dismissal.
+    virtual void MSWDismissUnfocusedPopup() { }
+
 private:
     wxWindow* m_owner;