views/month-cell: Replace overlay with box and empty bin

Since we don't make use of the overlay anymore, it's best to remove
it altogether.

This also changes the behavior of the widgets a bit: It adds an
empty AdwBin widget which expands vertically, allowing us to avoid
aligning the header and overflow button.

The pattern goes from reducible (where we deduct the overflow
height to retrieve the content) to additive (where we add the
overflow height to the content height), which requires us to
adapt GcalMonthViewRow with this new behavior.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/743>

src/gui/views/gcal-month-cell.c

@@ -44,7 +44,7 @@ struct _GcalMonthCell
 
   GtkWidget          *overflow_button;
   GtkInscription     *overflow_inscription;
-  GtkWidget          *overlay;
+  GtkWidget          *content_space;
 
   gboolean            different_month;
   gboolean            selected;
@@ -409,7 +409,7 @@ gcal_month_cell_class_init (GcalMonthCellClass *klass)
   gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, header_box);
   gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, overflow_button);
   gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, overflow_inscription);
-  gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, overlay);
+  gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, content_space);
   gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, temp_label);
   gtk_widget_class_bind_template_child (widget_class, GcalMonthCell, weather_icon);
 
@@ -538,8 +538,7 @@ gcal_month_cell_get_content_space (GcalMonthCell *self)
 {
   g_return_val_if_fail (GCAL_IS_MONTH_CELL (self), -1);
 
-  return gtk_widget_get_height (GTK_WIDGET (self)) -
-         gcal_month_cell_get_header_height (self);
+  return gtk_widget_get_height (self->content_space);
 }
 
 gint