Port to fontawesome4

We only have fontawesome-4 at fedora and this commit should update ipython to it.
After updating fontawesome to version 4 (or in my case, I replaced the bundled fontawesome with symlinks to the locally installed rpm version) and running `fab css`, anything should work as expected.

While porting, the icon `plus-sign` was not found, so I took only `plus`...
I have no idea (yet), where this code, that uses this `icon` property is :(