unique_window_class_test: managed to come up with some template magic to initialize all kinds of WNDCLASS structures
unique_window_class: class is now registered by ctor and unregistered by dtor.
unique_window_class_test: checks now whether the class is registered and unregistered correctly.
unique_window_class: support WNDCLASSEXA and WNDCLASSEXW only.

Support for these is sufficient, there's no need to use the old
versions. Both the production code and in particular the testcode become
simpler that way.