project: add Walkthrough.strings and LocalizedStringTableNames

Add Walkthrough.strings file for string localization
Add LocalizedStringTableNames struct for NSLocalizedStrings table
names constants

Not created a .string file associated to the storyboard to avoid
switching between multiple files if some strings are defined into
code and others into the storyboard.

Tuleap: #1399
Change-Id: Ie1e88a4413a773daef3be2eb95571d1b842274d0
Reviewed-by: Romain Bertozzi <romain.bertozzi@savoirfairelinux.com>