Import PSD Image Import for multiple layers

[audionerd]

Currently in new-sketchpane, dragging a PSD to Storyboarder works like this:

• PSD with a single flattened layer imports fine
• PSD with multiple layers and one layer named "reference" imports to Storyboarder's reference layer
• PSD with multiple layers where no layer is named "reference" will fail

Layers named 'main' or 'notes' are ignored.

We could instead try to insert named layers and flatten the rest to 'main', like we do for "Open in Editor" / Photoshop integration.

see: https://github.com/wonderunit/storyboarder/issues/1188

[audionerd]

Fixed by #2063. Now uses composite image instead of layers.