Reform scenario editor menu -> command mapping so that the non-platform-dependent code doesn't need to know where the menuitem is

src/tools/winutil.win.cpp

@@ -213,6 +213,10 @@ void beep() {
 	MessageBeep(-1);
 }
 
+void launchURL(std::string url) {
+	ShellExecuteA(NULL, "open", url.c_str(), NULL, NULL, SW_SHOWNORMAL);
+}
+
 // TODO: Implement modal session.
 // It seems that Windows doesn't have anything similar to the Mac modal session, so I might have to somehow simulate it myself.
 void ModalSession::pumpEvents() {