From Mattias Ellert:

Improvements to the doxygen filter
Properly handle macros in comments that uses /// at the beginning of the
line, i.e.

/// Begin_Macro
/// {
///   (macro code)
/// }
/// End_Macro

The old version only handled this properly if the macro was an external
file.

Don't trigger the filter if Begin_Macro and End_Macro appear on the same
line. These are comments explaining how the markup works, not markup.

Support NamespaceImp in addition to ClassImp. This makes the images for
e.g. TMath - which is a namespace and not a class - have proper names.