Photoshop CS5 adds support for layer groups whose nesting depth is
above 5.  The end markers of these groups use an undocumented
"lsdk" key for their section dividers, rather than the usual
"lsct".  This caused the PSD plugin to treat them as regular
layers, resulting in wrong layer-tree structure.

Add support for "lsdk" section dividers upon loading, and also
generate "lsdk" section dividers upon saving sufficiently-deep
groups.