Chromium: Add desktop entry.
Merge pull request #1970 from @ambrop72.

We already have a desktop icon from the browser wrapper, so this is only
for people who do not use the wrapper (for example if you don't want to
use Mozilla plugins).

Also, we someday might want to propagate the desktop item to the browser
wrapper as well.

Conflicts:
	pkgs/applications/networking/browsers/chromium/default.nix