[psd] Improve handling of Adjustment Layers without raster data

[davidetan]

The PSD import feature currently generates images bosed on layer raster data. Adjustment layers may or may not include raster data, depending on the software used to generate the PSD.

• Photoshop typically stores raster data for adjustment layers such as Fill or Gradient layers.
• Photopea, on the other hand, omits this data to reduce file size (see Photopea issue #5558).

At present, adjustment layers without raster data are ignored during import. This behavior might be unexpected for users, who should be informed of what occurred.

Goal

When importing a PSD file, if any adjustment layers are ignored due to missing raster data, display a dialog warning the user and listing the ignored layers.

Bonus

It would be ideal to support the import of Fill and Gradient layers even if they lack raster data:

• Fill Layers: These should be relatively easy to support.
• Gradient Layers: This might be more challenging, as Photoshop's gradient formulas are not publicly documented.