Not handle SpecificCharacterSet

[LingfaYang]

GetColumnData() in dcm_tree_model.cpp has:
std::stringstream value_ss; PrintValue(value_ss, data_element); column_data << QString::fromStdString(value_ss.str());
which may give wrong characters.
Please setCodec which is specified in SpecificCharacterSet tag.

[sprinfall]

Thank you for the comment.