Add PBD::to_/string_to specializations for ARDOUR::DataType
These could go into the data_type.h header but they don't really need to and it
means that ardour/types_convert.h can just be included by source files that
need to do type<=>string conversion. A potential problem with this is that if
all the specializations are contained in a single header then any class that
requires inclusion of that header to do serialization will be recompiled each
time types_convert.h is changed. I'm not going to worry about it at this stage,
it can always be broken up or improved upon later.
911e2af018