[2.3] - Wrap MODx.Layout regions in methods to ease "selective override"

A lot happens in the `MODx.Layout` constructor, making its override/customization (think custom manager theme) somehow harder than it should.

This PR aims to ease "selective override" (ie. overriding a single region while "inheriting" others).

It might not be the cleanest way to do so, but i'm open to suggestions/critics.