es-ES locale update

Tested and working. Just two things:
- In common_install.js I added a new line for the es-ES locale, but
maybe it can be donde with one line and the appropriate variable.
- I have not translated the description because I don't know how are you
going to manage it. This is the description translation for you to use
it:
"Añade botones de menú y de barra de herramientas para cambiar el agente
de usuario del navegador."