Model `week_starts_on` doesn't handle Sunday

[mcclurgm]

Prerequisites

• [x] I have searched open and closed issues for duplicates.

Describe the bug

In CalendarModel.vala, the first day of the week is set from Posix.NLTime.FIRST_WEEKDAY. The way that it's implemented doesn't work with weeks that start on Sunday (for example, in the US). From what I can tell (I can't find docs on FIRST_WEKDAY or nl_langinfo(_NL_TIME_FIRST_WEEKDAY), which I think is what this actually calls), langinfo uses Sunday = 1 through Saturday = 7. GLib uses Monday = 1 through Sunday = 7. So the method to subtract 1 from the Posix call works when the weekday is Monday or later, but Sunday will become 0. This is the enum value of BAD_WEEKDAY. Instead, it should wrap around to 7.

GNOME Calendar uses another call to WEEK_DAY1 to do this "wrap around" behavior. I've tried to implement this in Vala, but I can't get that value without a segfault, so I'm not sure how to fix this. FWIW, here's their code:

  union { unsigned int word; char *string; } langinfo;
  gint week_1stday = 0;
  gint first_weekday = 1;
  guint week_origin;

  langinfo.string = nl_langinfo (_NL_TIME_FIRST_WEEKDAY);
  first_weekday = langinfo.string[0];
  langinfo.string = nl_langinfo (_NL_TIME_WEEK_1STDAY);
  week_origin = langinfo.word;
  if (week_origin == 19971130) /* Sunday */
    week_1stday = 0;
  else if (week_origin == 19971201) /* Monday */
    week_1stday = 1;
  else
    g_warning ("Unknown value of _NL_TIME_WEEK_1STDAY.\n");

  week_start = (week_1stday + first_weekday - 1) % 7;

Platform Information

• [x] I'm using the latest version from git that I've manually compiled
• [ ] I'm using the latest released stable version

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