Method chaining in Buffer.
Added TODO.
Doxygen: ignore specific namespaces instead of whole directories.

Less maintenance burden with `#ifndef DOXYGEN_GENERATING_OUTPUT`,
removed the ones which are not needed anymore.
Removed unneeded `extern class`, added export macros to proper place.

Leftover from c4f321d91120a9cbd9269a0cf6e542447f4dd3f9, don't know at
all why and how that happen.
Doc++, TODO++