Issue #2852608 by Manuel Garcia, DyanneNova, markconroy, tim.plunkett, joelpittet, DickJohnson, Scorpid: Review layout CSS and markup

Issue #2857410 by tim.plunkett, DamienMcKenna: Adding a page-level #prefix or #suffix causes it to be output twice on one-col layout

Issue #2296423 by tim.plunkett, dsnopek, swentel, Manuel Garcia, tedbow, alexpott, xjm, andypost, dawehner, effulgentsia, Berdir, jhedstrom, catch, benjy, jibran, Wim Leers, tstoeckler, larowlan, webchick: Implement layout plugin type in core