Don't assume that the first monitor is always the primary one

This is not necessarily the case at least under MSW, so while it makes
sense to do this by default in wxDisplayImpl::IsPrimary()
implementation, MSW-specific version should override it instead of being
totally unused as it was until now.

src/common/dpycmn.cpp

@@ -159,7 +159,7 @@ wxString wxDisplay::GetName() const
 
 bool wxDisplay::IsPrimary() const
 {
-    return m_impl && m_impl->GetIndex() == 0;
+    return m_impl && m_impl->IsPrimary();
 }
 
 #if wxUSE_DISPLAY