[FEATURE] Implement loading for ZIP compressed PSD files

Now all types of 8 bit and 16 bit PSD files should load fine, including
multilayered ones. Saving should work as well.

STILL NEEDS TESTING:
We need to test if Photoshop will load 16bit multilayered files saved
in Krita. The point is we still save them differently from what PS is
used to.

CC:kimageshop@kde.org