Draft: WIP Add support for loading PSD descriptor data

This MR is a WIP for loading descriptor data from PSD's.

At this time only the data used for "new style" layer effects is fully interpreted and stored as json data. Currently only the drop shadow and solid fill layer effects are added if present. At this time the presence of the descriptor overrides all "old style" layer effects including ones we do not handle yet.

What more needs to be done:

• We want descriptor reading also available in the core, so that we can use it for Photoshop brushes, gradients, patterns, etc.
  • This needs conversion of the functions that read the basic data types from file because they differ between what is used for abr brushes etc, and for PSD images. I think it best if we adjust our plug-in code to do it similar to core, i.e. set endianness once and let the datatype reading functions handle conversion.
• We need the blend mode names as used in the descriptors, which is different from the names as used in other parts of a PSD.
• Handle other descriptor data types, especially those used for handling textual data.
• Add more loading functions that interpret the json data.