New es locale, a new locale entity, and a localization entity fix

Tested and working. I also added a new entity in every locale to
discriminate Pipelining group label and Pipelining checkbox label, to be
able to include extra info on the former and not to repeat it.
I also localized the Proxy Pipelining label which had a fixed value in the .xul file.
I included these, and no other extra changes, in tweak.jar

One last thing, you don't have links to this repo here:
https://addons.mozilla.org/En-us/firefox/addon/tweak-network/
nor here:
http://www.bitstorm.org/extensions/tweak/