99% of all mkenums uses in C libraries use the same basic template,
so add a mkenums_simple() function that takes care of everything for
us based on that template.
Features:
- optional function declaration decorator such as GLIB_AVAILABLE
- optional extra header prefix (e.g. for include needed for decorator)
- optional extra body prefix (e.g. for additional includes)
- optional function name prefix (e.g. to add leading underscores)
Fixes issue #1384
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.