layers.list: Add ability to filter by layer type

[tommoor]

Ideally implemented at the server level. This request came directly from a customer using the SDK, that it would be helpful to limit the data returned to, for example just symbols, or just artboards.

[bitpshr]

After initial investigation, this will most likely require an underlying change to the Abstract data format and the way files are cached on disk. Specifically, a layer's type should be cached along with its pageID for future retrieval.