:penguin: .desktop file without capital letter

Hello,

Thank you for developing and maintaining Atom!

I'm using it on GNU/Linux Ubuntu and it seems the launcher (`.desktop` file) has a capital letter in its name. It should be renamed to `atom.desktop` because a `.desktop` file should not contain capital letters as any other `.desktop` files that we can find in `/usr/share/applications`.

Note that we also have another problem because currently, the window's class of all Atom windows is `Atom Shell` but the launcher is named `Atom`. We should also add this line in your `.desktop` file:

    StartupWMClass=Atom Shell

Because without that, it's not possible for a dock/panel (like Cairo-Dock) to link a launcher (`Atom`) with its window (`Atom Shell`).

But it's certainly better to rename the window's class and this is why I just created this pull request in the `atom-shell` project! => https://github.com/atom/atom-shell/pull/698

Best Regards,

Matt
This input file is checked in here: https://github.com/atom/atom/blob/master/resources/linux/Atom.desktop.in

So it needs to be renamed as well in the repository.