Housekeeping tasks - clean up the code

Since I didn't have time to do the kind of clean up I mentioned in !1572 (comment 7934286), I decided to create an issue to track the progress.

• use #pragma once everywhere
• remove pointless comments like // ==========..., etc.
• fix Doxygen comments
• verbatim include private headers used in a single translation unit (see !1811 (comment 10049951))