Load all translations with changeLocale

Currently, Localization::changeLocale loads only the core translations:
packages and site translations have to be loaded manually.
It's better to load all translations when switching the current locale,
since all the localization files should be considered as a whole.
Merge branch 'load-all-translations-on-locale-change' of https://github.com/mlocati/concrete5 into mlocati-load-all-translations-on-locale-change