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
22c18a107e