jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
My recollection (which is fuzzy and could be wrong!) was that we couldn't rely on getting a change notification for each layer for which the index has changed as a result of a change of layer structure. This is why I'm so hesitant to track `index` as part of the state of the DOM. Maybe I'm wrong and we *always* get a change event for each layer whose index changes. But I'd like confirmation of that before adding this state. In the short term, it seems safer to just walk over the layers to find the bounding indices, or again something like adobe-photoshop/generator-core#258.