This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Replaces map_to_relative_path() from Mapper.cxx.

Prepare for the plugin interface.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.