utils: Turn number of weekdays into a macro

While "7" may be obvious in the context of calendaring that
it represents the number of week days, it often gives me the
impression of it being a magic number when I'm hyperfocused on certain
parts of the codebase for several consecutive hours. Turning the
number into a macro helps me scan functions much more easily.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/674>

src/gui/event-editor/gcal-alarm-row.c

@@ -25,6 +25,7 @@
 #include <glib/gi18n.h>
 
 #include "gcal-alarm-row.h"
+#include "gcal-utils.h"
 
 struct _GcalAlarmRow
 {
@@ -63,7 +64,7 @@ format_alarm_duration (ICalDuration *duration)
   guint hours;
   guint days;
 
-  days = i_cal_duration_get_weeks (duration) * 7 + i_cal_duration_get_days (duration);
+  days = i_cal_duration_get_weeks (duration) * N_WEEKDAYS + i_cal_duration_get_days (duration);
   hours = i_cal_duration_get_hours (duration);
   minutes = i_cal_duration_get_minutes (duration);