[2.3] - Adding MODx.Layout methods to "streamline" its override/customization

This is a first step to ease MODx Layout (an [Ext.Viewport](http://docs.sencha.com/extjs/3.4.0/#!/api/Ext.Viewport) derivative class) customization.

Since manager themes could override the default behavior/content using `MODx.Layout`, i think we should provide some kind of API so third party components could rely on it, without breaking everything if the manager theme is not the default one.

I would love to get feedback from people building an "heavily" modified manager theme (ie. not just CSS changes)