Improve get_strings

get_strings.php is the script used to populate language files (lang/xx/language.php) with English strings collected from the source code so that translators can work on it.

Below is a list of proposed improvements.

• Remove unused strings. The current version of get_strings.php does not remove from language.php strings that are not used anymore. This leads to translators wasting time translating useless strings and imprecise stats (different language.php have a different total number of strings). If there is any reason to keep unused strings we should do it in another file.
• Reorganize (rewrite?) the script so that it is possible to unit test it. Making easier to catch and fix regression bugs like this one.
• Decide whether the script should order the strings alphabetically or as they appear in the source files (current behavior).
  • Alphabetically make it easy to catch duplicates
  • As they appear in the source files is better when you are translating as they are organized by feature and you have an idea of their context