refactor: rename the atom directory to shell

shell/app/atom_main_delegate_mac.h

@@ -0,0 +1,15 @@
+// Copyright 2013 Slack Technologies, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_APP_ATOM_MAIN_DELEGATE_MAC_H_
+#define ATOM_APP_ATOM_MAIN_DELEGATE_MAC_H_
+
+namespace atom {
+
+// Initializes NSApplication.
+void RegisterAtomCrApp();
+
+}  // namespace atom
+
+#endif  // ATOM_APP_ATOM_MAIN_DELEGATE_MAC_H_