rename layers-2d to reference-layers

[kfarr]

why? 2d layers is not a great name, instead "reference layers" is more accurate to depict layers that are references of real-world maps, 3d scans, 2d engineering drawings, etc.

new name:
dom id: "reference-layers" instead of "layers-2d"
data-layer-name for users: "Reference Layers"
js variable name referenceEntities instead of layers2DElement

[kfarr]

closed by #326