Expose XWkiDocument#getTranslationLocales() and XWiki#getAvailableLocales() to scripts

This would make it easier to work with page translations. It would allow us to replace:

$xwiki.getXWikiPreference('languages').split('\s*[|, ]\s*')

with:

$xwiki.availableLocales

and remove the need to "convert" the string to locale in order to properly display the locale name.