This feature defaults to off, and can be turned on for individual windows,
or (with for_window) for all new windows. See the userguide change.
This commit is partially based on work by:
• Marius Muja
• mickael9
• Esteve Varela Colominas
• Bernardo Menicagli
This works better with meson, where .h files can be declared as being part of an
executable easily, but I couldn’t find a way to declare
e.g. include/atoms.xmacro as a dependency.
related to #4086
