Align source layer implementation with QGIS

[gjmooney]

This issue to to track how we should handle sourceLayer stuff after a conversation with @martinRenou

QGIS organizes their vector tile layers like:

• project layers
  • map layer
    • renderer
      • styles
        • style
          • symbols
            • symbol
              • symbol layer
                • option
                  • color stuff

The type and sourceLayer from our schema that the style is being applied to is stored in the style part of their xml.

To bring our schema more in line with the QGIS implementation, I was thinking we could basically combine type, sourceLayer, and color entries from our schema into a style object, and have an array of those in the schema.

This would require a bit of refactoring, and since this is only relevant to one layer type it might not be worth the time expenditure at the moment, so I'm just opening this issue for now to keep it on our radar.