Display leap day anniversaries every year on calendars

[SNoiraud]

Fixes #12511

Birthday on February 29th only taken into account every 4 years
in the calendars

Added an option in the preferences display tab
If checked, we display the birthday on march 1 for non-leap years
If not checked, we display the birthday on february 28 for non-leap years
When we have a leap year, display the date on february 29

[emyoulation]

Ummm. Kinda curious if it handles leap centuries... since we just had one in 2000.

[SNoiraud]

Kinda curious if it handles leap centuries... since we just had one in 2000.

I use the isleap method from the calendar library. I expect the standard function is correct about centuries and multiple of 400 years.

[Nick-Hall]

@SNoiraud The duplicate code needs to be put in a utility function. There is also a conflict to resolve.

[Nick-Hall]

@SNoiraud I suggest a new anniversary(year) date method, to return the day and month of the anniversary of a date in a given year.

For example: Date(1904,2,29).anniversary(1910) would return (28, 2), (1, 3) or (29, 2) depending on the preference settings.

I have added a commit with the function and associated unit tests. Run them with:

GRAMPS_RESOURCES=. python -m unittest gramps.gen.lib.test.date_test.AnniversaryDateTest

Let me know if this is OK.

[Nick-Hall]

Rebased.