Combine layer metadata into single file

[crisner]

Fixes #371 (<=== Add issue number here)

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[crisner]

Because this involves changes in different parts of the project I feel it would be better to wait to merge this until LEL 2.2.0 is published so that we can contain the bugs if any within the LEL repo. However, this is ready for review. Thanks!

[crisner]

@nstjean, could you check if the layer menu is working as expected? Thanks!

[crisner]

Ah. Looks like some tests are failing. Shall take a look.

[crisner]

The test failed because we had changed the naming for the layers when combining the JSON files, which had affected the layer ids. Fixed.

[nstjean]

This looks great!! Works well for me!