Allow use of JLayouts in Plugins

Currently it's pretty hard to build layout overrides for plugins - so the obvious solution would be to use JLayouts to allow easier overrides of the layouts coming out. As most of these aren't going to be used between plugins it makes more sense to bundle them with the plugins rather than in the generic Layouts folder - hence this PR.

This PR implements this search in the path search and also gives a demo use in part of the pagebreak plugin (just the bit for the next/previous buttons - not elsewhere as this is just a Proof of Implementation)