Add rudimentary mash shelf functionality.

mash/shelf/main.cc

Index: mash/shelf/main.cc
diff --git a/mash/system_ui/main.cc b/mash/shelf/main.cc
similarity index 75%
rename from mash/system_ui/main.cc
rename to mash/shelf/main.cc
index 0c5832bea3747036a0145bc7a583b62099762ddb..636e0e0c1bbb1e1b513afd08b4e4d46d4939e2dc 100644
--- a/mash/system_ui/main.cc
+++ b/mash/shelf/main.cc
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "mash/system_ui/system_ui.h"
+#include "mash/shelf/shelf_application.h"
 #include "mojo/public/c/system/main.h"
 #include "mojo/shell/public/cpp/application_runner.h"
 
 MojoResult MojoMain(MojoHandle shell_handle) {
-  mojo::ApplicationRunner runner(new mash::system_ui::SystemUI);
+  mojo::ApplicationRunner runner(new mash::shelf::ShelfApplication);
   return runner.Run(shell_handle);
 }