Let's allow packages to setup the active locale

### THIS IS JUST A THINK-ABOUT PULL REQUEST

The file /concrete/config/localization.php defines some locale-dependant constants.
But at that moment the active locale is the website default one, not the page-specific one.
Let's ask the installed packages to set the ACTIVE_LOCALE on a per-page basis.

Related multilingual pull request: https://github.com/concrete5/addon_internationalization/pull/39