continue fixing nonfree elements
fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.
Merge pull request #3163 from StevenPuttemans:fix_nonfree_items
remove structured_edge_detection.hpp from blacklist
don't discard pure virtual methods in hdr_parser.py (was broken anyway)