For #111 : dynamic layers in Wegue format

[justb4]

Introduces a new Wegue Layer type LAYERCOLLECTION which is mainly a URL pointing to a remote endpoint (or file local to the Wegue App) that contains an array of Wegue Layers (which in theory could even again contain LAYERCOLLECTIONs).

• As the loading of the URL (via fetch()) is asynchronous, several functions are now async/await (ES2015 syntax).
• test added, NB final version needs to point to Wegue GH link

Later similar "expanding layer types" could be added for WMS, WFS etc (via GetCapabilities) and even OGC OpenAPI Features ("WFSv3").

[chrismayer]

Thanks for this @justb4. Two questions:

1.) Why was it necessary to delete test/unit/specs/ol/Map.spec.js
2.) Any reason you removed the alias me for this in the existing code? Creates a lot of unrelated diffs in your changeset.

[justb4]

@chrismayer sorry, your questions escaped me.

ad 1) test/unit/specs/ol/Map.spec.js was a duplicate of test/unit/specs/components/ol/Map.spec.js.

ad 2) me = this is uneccessary in ES2015 with arrow functions

Ok strictly not part of issue, but cleanup which I considered too minor for issue/PR.

[chrismayer]

Thanks for clarification.

One further point: Due to asynchronous handling the position in the mapLayers array is not resprected for type LAYERCOLLECTION. In the app-conf.json the demo layer for a LAYERCOLLECTION is laced somewhere in the middle of the mapLayers array but is rendered topmost in the layer stack.

[justb4]

Hmm, yes. The Layers coming from the LAYERCOLLECTION are reversed as to have them internally added bottom to top just like the regular Layers. But yes, the async nature could lead these always added as last/topmost. Tough problem to solve, if ever a problem in practice: 1) usually the entire app is configured from a single LAYERCOLLECTION or the latter is a series of overlay Layers on top of regularly configured baselayers (Geolicious use case).

Solution would be to "stack" any remaining regular Layers (not adding them) after the LAYERCOLLECTION and add them after the async finishes. Would be interesting with multiple LAYERCOLLECTIONs....

As an alternative, for progress, we could merge this PR and open a new issue for "LAYERCOLLECTION messes up Layer stack ordering"...