Clean up

* Remove unused code
* Unify multiply-defined vars
* DRY up duplicated code
* Add missing semicolons
* Expand one-line functions
* Expand single-line objects
* (Visual Studio removed trailing whitespace and normalized some spacing)
Simplify and clarify code in AgsLoader

* Rename variables representing part of layers.json to contain "Config" (e.g. folderConfig, serviceConfig)
* Simplify "whitelist" code
Simplify and clarify folder loading
Eliminate guids; simplify and clarify service loading

Guids were only being used to connect a service spec to its "config" properties (from layers.json).
It's simpler to just attach the relevant config object to the service spec.
Simplify and clarify

* Eliminate duplicated code
* Don't pass "source" to AgsLoader
* Rename "currentService" as "serviceConfig"
Split loadFolders() into smaller functions
More simplification
Simplify and clarify code in WmsLoader.js

* use variables for _makeContainerNode etc. (like AgsLoader does) to avoid passing them around multiple times now that this module is more complex
* Separate loadCatalogUsingResourceInfo() function
* Factor out duplicated code
* Restore WMS layers in layers.json for testing
* In layers.json schema, all "extent" fields are required
* Renaming
* Move "GetCapabilities" comment where it makes sense