[GNC-dev] Configurable Year Start Day/Month

I'm seeking comments on a proposal to support a configurable year start
day/month. This goes back to
https://lists.gnucash.org/pipermail/gnucash-devel/2021-September/045946.html
but I see the issue a bit clearer now.

Firstly, GnuCash already supports Accounting Period Start/End Date eg
https://github.com/Gnucash/gnucash/blob/4.12/libgnucash/app-utils/date-utilities.scm#L621-L625
(Note: even code variable names demonstrate the confusion between
accounting and fiscal year)

Accounting Period only supports a single period changed manually which is
very limiting compared to Previous/This/Next Year variants automatically
updated each day as supported elsewhere in GnuCash and other packages.

I've hacked a different start year month into each version over the last
couple of years which could be modified to change day:
https://lists.gnucash.org/pipermail/gnucash-user/2021-October/097880.html
On year-start day/month (ie new year) the year is numbered by the calendar
year at the end of the period (eg 1 Jul 2020 to 30 Jun 2021 is 2021).
Reporting by year variants works well (although I don't use Business
Features) but the change process is inconvenient and too complex for most
users.

If the changes are likely to be accepted I'm happy to:
* do any changes required as described in
https://wiki.gnucash.org/wiki/Custom_Reports
* update the documentation to explain the preference setting
* rename variable: cal-year to cur-year
* add a previous 2Year Start/End period so preparing reports for the
previous year allows the year before that to also be accessed

I understand the GnuCash Preferences, Report, year-start day/month would
need to be in a compiled executable. I'd ask someone to send me one as a
windows executable. A default of 1 January would ensure users are not
affected unless they change the setting.