Fix crash if "@return" Doxygen tag was used on a node without any return
type (such as a class, for example). Ignoring it might not be the best
thing to do, but it's definitely better than crashing and it's not
really clear what else could be done anyhow.
Closes#1516.
Source/DoxygenTranslator/src directory is renamed Source/Doxygen
Renamed files in this directory to short names using lowercase as is the
convention for the rest of the SWIG source.
C++ extension is also .cxx like other SWIG source code.
I used doxy as the prefix for most file renames because without this
Doxygen/parser.* would be easily confused with CParse/parser.* so
Doxygen/doxyparser.* is renamed from DoxygenTranslator/src/DoxygenParser.*
