Invalidate wxDisplay cache under MSW when the displays change

The cache added in 990c8bfd73 was not
invalidated properly, meaning that wrong information was returned when
displays were [dis]connected after the application startup.

Fix this at least for MSW by invalidating the cache on receiving
WM_DISPLAYCHANGE (which means that sometimes we will do it
unnecessarily, as the change in resolution of an existing display
doesn't require cache invalidation, but this shouldn't be a big problem
in practice as the speed with which the user can change the display
resolution is not very high).

Closes https://github.com/wxWidgets/wxWidgets/pull/1090

include/wx/display.h

@@ -115,6 +115,11 @@ public:
     void ResetMode() { (void)ChangeMode(); }
 #endif // wxUSE_DISPLAY
 
+    // If the implementation caches any information about the displays, calling
+    // this function clears it -- this should be done e.g. after a display
+    // [dis]connection.
+    static void InvalidateCache();
+
 private:
     // returns the factory used to implement our static methods and create new
     // displays