fix(layout): limit region scope to the layout element

The idea here is to fix the region scope to the layout container. 

The difference from #1685 is that the regions $el are "rescope" after layout has been rendered and not during initialization, so we are sure that layout.$el is defined.

The trick is to override the `getEl()` implementation to use Backbone.$ only as a fallback and to use the `this.$el.find` as it's already done in `Backbone.View`.

related to #1685
I haven't really tested it to be honest, but it works in my head. No bias, but I'd say its significantly clearer then the previous attempt and the one in this PR

(sorry, this was the diff I meant to link https://github.com/megawac/backbone.marionette/commit/525bc2d36913db76e7ac1f38e61ec9f1b54907f7)