add support for product-specific strings

EdgeCode would like to add product-specific, localizable strings.  Instead of having to duplicate the entire src/nls/root/strings.js file, we'd like to overlay just a subset of strings.  This change adds this extensibility with the following changes:

1. introduce src/nls/root/strings2.js which defines product-specific strings.  For Brackets, this file just contains the APP_NAME string.
2. edit existing src/nls/root/strings.js to remove the single APP_NAME string
3. add code to src/strings.js which includes strings2.js and appends any product-specific strings to the 'strings' collection