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/common/dpycmn.cpp

@@ -113,6 +113,11 @@ wxDisplay::wxDisplay(const wxWindow* window)
     return Factory().GetFromWindow(window);
 }
 
+/* static */ void wxDisplay::InvalidateCache()
+{
+    Factory().InvalidateCache();
+}
+
 // ----------------------------------------------------------------------------
 // functions forwarded to wxDisplayImpl
 // ----------------------------------------------------------------------------
@@ -229,13 +234,15 @@ wxSize wxDisplayImpl::GetPPI() const
 // wxDisplayFactory implementation
 // ============================================================================
 
-wxDisplayFactory::~wxDisplayFactory()
+void wxDisplayFactory::ClearImpls()
 {
     for ( size_t n = 0; n < m_impls.size(); ++n )
     {
         // It can be null, that's ok.
         delete m_impls[n];
     }
+
+    m_impls.clear();
 }
 
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)