Fixed doxygen warnings

[snej]

• Removed obsolete properties from Doxyfile
• Removed a \ref in Expert.hh that doxygen couldn't identify because it's to a C function, and the Doxyfile_C++ doesn't include the C headers.
• Made Xcode run doxygen if either Doxyfile has been touched.
• Fixed Xcode 15 settings nags, while I was at it