Decided to panic in the hwnd sanity check in sysdata_windows.go; the situation /is/ exceptional, after all (and we need to just get rid of all errors sometime).
Quick changes: turned "Window has been opened" to "Window has been created" since I'm going to separate creating and opening (Open() will call Create()).
Big change to Window: split Open()'s functionality into Create() and Open(); they no longer return errors.