Let's allow packages to setup the active locale

[mlocati]

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: concretecms/addon_internationalization#39

[mlocati]

This is a potential alternative to concretecms/concretecms#1381

[mkly]

These are interesting ideas. I'm going to take a closer look early next week.

[Remo]

This PR would change a small behaviour. The language returned by $u->getUserDefaultLanguage() currently wins over the multilingual add-on. That means that every part of a site is currently in Italian if you have selected it as your default language. When you load the multilingual add-on earlier, it would win over $u->getUserDefaultLanguage().

To me, this "new" behaviour would even be better, but I guess that's up for discussion. It's what I currently have in our custom fork.

Background: You have a site with a couple of forms [t() strings in the frontend output]. When I now select "German" as my default language, I won't be able to see the correct form block output if I navigate to /it/form..

I thought about filing a bug report for this but you know...

In short: I'd prefer what this PR does over the current behaviour but it's different. Just thought I'd point it out..

[Remo]

@mlocati I'm going to close this, the one that imho matters the most at this point is concretecms/addon_internationalization#47 Let's see if we can get it merged..