WIP
Refactoring of context and layer stuff

Conflicts:
	src/models/Model_DemWithOverlays.js
	src/renderer/effects/texture-blend.js
Added sorting functionality to TextureBlend effect
Added Backbone.Events mixin to RBV.Context
Derive RBV.Context from 'global' context and base Model_DemWithOverlays on RBV.Context events (this is a starting point, not finished yet)