quit app on linux/windows if the main window is closed

[ghost]

as i understood electron should quit the app automatically if all windows are closed. maybe hidden is not closed. i dont know, but this works well.

Fix for #228

[hql287]

@jens-t Thanks for the PR, just want to clarify this:

On macOS, the application will only quit if you press CMD+Q or select Quit App from the menu bar, so this is for Linux (probably Windows as well), right? If yes then I think we'll need a few conditional to make sure it behaves correctly on macOS.

[ghost]

Ok, makes sense. Didn't know that this is unwanted behaviour under osx. So i would say a simple condition for each of the 3 lines that checks that the platform is not osx would do it.

I add a commit for this later today.

[ghost]

Changed the behavior as discussed in #233. Ready to merge :)
(Devtools are closed by the quit action)