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/gcal-window.c

@@ -126,7 +126,7 @@ struct _GcalWindow
   GtkWidget          *quick_add_popover;
 
   /* day, week, month, year, list */
-  GtkWidget          *views [6];
+  GtkWidget          *views[N_WEEKDAYS - 1];
   gboolean            subscribed;
 
   GcalContext        *context;