Add uncompressed 16-bit depth PSD format support

Since uncompressed color depth == 16 PSD file is very common to see, this commit adds support for this type of PSD file.

This patch can partically resolves https://bugs.kde.org/show_bug.cgi?id=428238 . I also notice that krita support much wider range of PSD format than kimageformat, not sure if it's possible to port the krita PSD parsing code to kimageformats (but anyway this will not be done in this Merge Request).

Note: I can't find any compressed PSD file with depth is 16, one of my friend says 16 bit depth PSD file doesn't allowed to use compressed. I don't have too many PSD file to test, so please review and test if possible. Thanks!