Add "New Session" context menu item to launcher

[rpwoodbu]

It looks to be possible to add context menu items to the launcher icon for the app:

http://developer.chrome.com/extensions/contextMenus.html#method-create

Currently, due to behaviors on Chrome OS, the "New Session" window (mosh_client.html) is left open, and if the user closes it, he may never get it back. Other platforms will bring the window back when the user attempts to launch the app again (onLaunched() fires). But just because of Chrome OS, I'm leaving this "New Session" window open but minimized, which pollutes the workspace, and isn't a great UX.

By adding a context menu to the launcher icon, I could just close that window once the session is started, and let the user right-click the app icon to start a new session. This is similar to how the Secure Shell app works.

This may only show on Chrome OS, but that's OK, as that is the only platform AFAIK that needs this.

[rpwoodbu]

Done in SHA 6e6ffec.