linux: Rename window's class to Atom

The window's class should be Atom instead of Atom Shell because the
launcher and the binary to launch Atom are called 'atom' and not 'atom
shell'. This is why currently all Atom's windows will not be linked to
their launcher in a dock (e.g. with Cairo-Dock).

Note that it's not advised to add white-spaces in a window's class
('Atom Shell').
Merge pull request #698 from matttbe/master

linux: Rename window's class to Atom