Added missing keybind in linux.cson.

`'ctrl-,': 'application:show-settings'` exists in `win32.cson` and `darwin32.cson`, it makes sense to exist in `linux.cson` too.

Question is how are the menu bars updated? 

It updates fine if the above keybind is in the user's keymap.cson, but it doesn't update if it's in linux.cson. (done after rebuild/reinstall) 


**Unrelated to pull request**:

Suggestion: `ctrl-alt-o` should be binded to `application:open-folder`. It's useful and many people will benefit from having this by default.


Discussion: however, *ctrl-alt* doesn't make the most sense, *ctrl-shift* does. As we (kind of) want to open multiple files (implicitly a folder) and that's usually what *shift* key is related to. However, `'ctrl-shift-o': 'application:open-dev'` and `'ctrl-shift-i': 'window:toggle-dev-tools'` exist already. 

If we really want to use `ctrl-shift-o` as open folder, it wouldn't make the most sense to have `ctrl-shift-i` as dev tools. We must change both *open-dev* and *toggle-dev-tools* to be combined with *alt* instead of *shift*. What do you think of this?