Initial native menus implementation

@redmunds 

This is the initial implementation of native menus. 

For review, I'd recommend starting with `appshell_extensions.js`, since that file defines the API's for adding, removing and modifying native menus. 

Selecting a menu item will dispatch the associated command to the JavaScript application.

Note that this pull request only adds the APIs, and does not change the menu layout in Brackets. A subsequent checkin to the Brackets branch will add unit tests for these APIs, and an additional pull request will migrate from our HTML menus to native menus.