Bugfix: _NET_SUPPORTED property fixes

Fix a typing mistake that may cause _NET_SUPPORTED root property to
become corrupted.

This bug was introduced in:

Implement EWMH number of desktops property

http://code.stapelberg.de/git/i3/commit/?h=next&id=b47f480728a6d053c117e4b99ee4c78d99ba6a71

Additionally adds one missing atom to _NET_SUPPORTED.

fixes #1298