gcal-date-time-utils: Fix week start

In locales where the first weekday is a monday, the previous code
had a off by one error determining the week a sunday is in.
The problem ultimately arose because in the
`gcal_date_time_get_start_of_week()` function the variable
`n_days_after_week_start` would become negative (-1 in this case) making
the following `g_date_time_add_days (date, -n_days_after_week_start)`
add 1 day resulting in `start_of_week` being set to the start of the
following week.

The solution was to make sure `n_days_after_week_start` would be
positive by adding 7 to it. The variable is taken modulo 7 hence this
should not introduce any bugs.

Fixes #583


(cherry picked from commit 07f544b7)
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