Creates a hidden window with supplied window proc registering the class for
it if necesssary (i.e. the first time only). Caller is responsible for
destroying the window and unregistering the class (note that this must be
Creates a hidden window with supplied window proc registering the class for
it if necesssary (i.e. the first time only). Caller is responsible for
destroying the window and unregistering the class (note that this must be