Fix building with wxUSE_STL enabled in VS2019 16.6
Exporting standard C++ library types is not allowed any more, see
https://developercommunity.visualstudio.com/content/problem/1042081/142628801includexhash201332-error-c2338-this-funct.html
so don't use DLL export declarations for wxHash{Map,Set} classes, which
derive from std::{map,set}.
Since the classes are header-only, this should have no direct
consequences. Only classes that extend wxHashMap or wxHashSet, like
wxImageHistogram, have to be made header only as well.
Remove the wxLogError message in wxImageHistogram::FindFirstUnusedColour, so
log.h and translation.h do not have to be included in the header.
Closes https://github.com/wxWidgets/wxWidgets/pull/2009
590e32c60b