add a layout_pathname() method

The goal is to encapsulate the template-as-filename
logic in view_pathname and layout_pathname such that
template systems that don't use files (like TemplateDeclare)
will have an easy way to overload them.

WARNING: I don't know if that's the right solution. It makes sense to me, but I've changed the method view() to view_pathname() so that it can have its sister method layour_pathname(). Feel free to alter to your taste, if you so desire. :-)