Consolidate locale strings

[vrillusions]

Right now there's three different files which isn't needed for something this simple and causes some repeats. Can all be moved to a single DTD file included everywhere. See https://developer.mozilla.org/en-US/Add-ons/Overlay_Extensions/XUL_School/The_Essentials_of_an_Extension for more info on this.

May want to have two files. One with all the application strings and another with the skin names. Should make it easier when creating new skins to edit the one file and copy it over to other locales.

Also may rename some IDs to have proper namespacing and to eliminate ambiguity.