History: Improve get_strings

{REMARKSBOX(type="warning" title="Warning")}All information for end users to be able to translate should be in a workspace on tiki.org{REMARKSBOX}

^This page is kept for historic reasons. get_strings.php was rewritten in Tiki 8 following the points listed below.^

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 [https://dev.tiki.org/tiki-view_tracker_item.php?itemId=3881|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