date-chooser: Clamp to first day of month when clicking previous/next buttons

When clicking the previous/next buttons, the view might not be aligned
to the start of the month, which can cause confusion.

Clamping the date to the first of the month removes this confusion by
being consistent. This also matches the behavior of the previous/next
buttons in the main toolbar.

Closes #1606

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/821>

src/gui/common/gcal-date-chooser.c

@@ -599,6 +599,9 @@ gcal_date_chooser_set_date (GcalView  *view,
   g_date_time_get_ymd (self->date, &y1, &m1, &d1);
   g_date_time_get_ymd (date, &y2, &m2, &d2);
 
+  if (y1 != y2 || m1 != m2)
+    d2 = 1;
+
   gcal_set_date_time (&self->date, date);
 
   if (y1 != y2 || m1 != m2 || d1 != d2)