[O] [RFC] Change some defcustoms into defcont

[Nicolas Goaziou]

Hello,

I'd like to suggest changing the following defcustom variables into
defconst:

  - org-deadline-string
  - org-scheduled-string
  - org-closed-string
  - org-clock-string
  - org-comment-string
  - org-quote-string
  - org-effort-property
  - org-archive-tag

There's no fundamental reason to change these, as they belong to Org
syntax, much like star character for headlines and ":END:" closing
string for drawers.

They can also introduce bugs in code when modified (e.g. modifying
`org-effort-property' breaks `ox-taskjuggler'). Of course, this can be
avoided with careful checks, but this puts an extra burden on developers
for a tiny benefit for the user.

Eventually, they can be costly, since changing any of them implies that
cache in every Org buffer must be erased.

Allowing customization is generally good, but I think Org syntax should
never be a moving target.

Note that this shouldn't introduce much backward incompatibility (unless
user changes them through customize interface) since a defconst can
still be setq'ed and variables will still be used in code base.

Opinions?


Regards,

-- 
Nicolas Goaziou