Expose sourceLayerIdentifier on Layer

[tobrun]

A useful addition to our current Layer API is exposing sourceLayerIdentfier, while we use it internally we don't expose a getter for it. Such a getter can be useful to filter layers. For example take traffic, you want to add a traffic layer just above all other road layers. For this you would need to get all the style layers and filter out the ones with sourceLayerIdentifier="road".

[tobrun]

Alternatively to exposing a getter we could also solve this use-case with a getLayers(String sourceLayerIdentfier) this might be a bit cleaner as we don't require the user to get all layers and do the filtering themselves.

Thoughts @ivovandongen?

[ivovandongen]

@tobrun Let's start off with a getter and see if there are actual use cases for a filtered getter. If that's the case, I'd like to make it a little more extensible with a filter object that can have a number of properties to filter on (like the source name).

[tobrun]

Will work on adding the getter, 👍 on the filter idea.