Fix qualifier parsing in SwigType_add_qualifier()

- use list to ensure qualifiers are unique and sorted
- now allows 'qual' to contain multiple qualifiers
Fix SWIG's handling of qualified (e.g. const) variables of array type
Cosmetic changes in SwigType_add_qualifier
Uncomment testing of overloading of const char arrays which was fixed in svn r12541 for swig-2.0.3.