Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

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

Issue #2509218 by plach, stefan.r, pwolanin, effulgentsia, dawehner, jhedstrom, almaudoh, Wim Leers, subhojit777, iMiksu, alexpott, catch, xjm, lauriii, joelpittet, Cottser: Ensure that SafeString objects can be used in non-HTML contexts