Clean up default language handling

Drupal has a little utility function called language_default(). This uses a variable as in variable_get() to retrieve the default language. Other places save this variable when the default language is updated. The reason for having this function at all and using a variable for it were as follows:

(1) this function and the variable is available outside of locale module, so we have information on a default language even if locale module (and therefore language list) is not there
(2) there is a single place where the default language is stored, its not a flag in the database, where it seemed to be harder to ensure only one of the languages is the default (also see (1) on when no db is available)
(3) there is no quicker way to get a default language then using the variable system

There are also several problems with the variable:

(1) As @sun pointed out in #1265700: Little modernization of language_default(), its storing a whole object in a setting, not nice.
(2) Related: duplicates data from the language table, can become outdated independently, and needs some special attention to keep updated.
(3) Langauge objects do use the "default" property to cache a boolean on whether they are the default, and the load and save functions need to work with updating the variable based on that property, so data storage structure is pretty different to the objects in PHP

How can improve on this situation?

Parent issue

#1260488: META: Introduce real language APIs