Issue #2576533 by alexpott, stefan.r, Wim Leers, dawehner, xjm, effulgentsia, catch: Rename SafeStringInterface to MarkupInterface and move related classes

Issue #2562155 by stefan.r, joelpittet, alexpott: Cast TranslationWrapper objects in config to string during Config::set()/setData()

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2040135 by Wim Leers, Berdir: Caches dependent on simple config are only invalidated on form submissions