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

src/msw/window.cpp

@@ -58,8 +58,9 @@
     #include "wx/ownerdrw.h"
 #endif
 
-#include "wx/hashmap.h"
+#include "wx/display.h"
 #include "wx/evtloop.h"
+#include "wx/hashmap.h"
 #include "wx/popupwin.h"
 #include "wx/power.h"
 #include "wx/scopeguard.h"
@@ -4692,6 +4693,8 @@ bool wxWindowMSW::HandleSysColorChange()
 
 bool wxWindowMSW::HandleDisplayChange()
 {
+    wxDisplay::InvalidateCache();
+
     wxDisplayChangedEvent event;
     event.SetEventObject(this);