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/popupwin.h

@@ -151,6 +151,9 @@ public:
                                   WXWPARAM wParam,
                                   WXLPARAM lParam) wxOVERRIDE;
 
+    // Override to dismiss the popup.
+    virtual void MSWDismissUnfocusedPopup() wxOVERRIDE;
+
 private:
     void DismissOnDeactivate();