Added linux open-folder keybind.

Moved `application:open-dev` to *ctrl-alt-o*.

Now, should `window:toggle-dev-tools` remain on *ctrl-shift-i* or be moved to *ctrl-alt-i*, which would make more sense if other dev keybinds are combined with alt.

![dev_tools](https://cloud.githubusercontent.com/assets/6031925/4074658/1bc1b2bc-2ea4-11e4-8c42-7f7b7f0db67e.png)
Note: in the screenshot, Atom isn't prebuilded yet.