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 would drop the `fa` class here and add it to the code that use the properties `icon` to add the icon to whatever it adds it to.