Support automatically hiding the menu bar

Adds a `core.autoHideMenuBar` config setting to auto hide the menu bar on Linux and Windows. `Alt` will bring show the menu bar in this mode.

![auto-hide](https://cloud.githubusercontent.com/assets/671378/5079234/493f802c-6e67-11e4-9614-4ab547197e54.gif)

This new command `window:toggle-menu-bar` will also be available from the command palette and the `View` menu.

Closes #2914