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

Also: remove console.log() calls