- Store the Layer ID (lyid) block. Use GIMP's layer tattoo as a PSD
  layer ID, hence mirroring PSD load processing (we were already reading
  this block into our layer tattoos but always exporting with no ID).
- Add support for the Layer Selection ID(s) block (0x042D) both on
  import and export in order to store and restore the multi-layer
  selection.
  We were previously using the Layer state information block (0x0400) to
  store the active layer, but it doesn't seem to be usable for multiple
  layer selection. Actually it is even doubtful if this was working fine
  even for single layer selection but I can't be sure (I could only test
  in non-Photoshop software available to me). So the new logics is:
  * If more than 1 layer is selected, store only the Layer Selection
    ID(s) block.
  * If exactly 1 layer is selected, store both the Layer Selection ID(s)
    and Layer state information blocks.
  * Otherwise (no layers selected) do not store any of these blocks.