views/month: Set all non-visible rows as unfocusable

When tabbing into the month view, the keyboard focus would go
to the first week row in the widget hierarchy, rather than the
visible row. Setting the `GtkWidget:can-focus` property to
FALSE on every non-visible row only allows keyboard focus
within the visible rows.

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

@@ -498,7 +498,13 @@ update_row_visuals (GcalMonthView *self)
 
   for (gint i = 0; i < self->week_rows->len; i++)
     {
-      GcalMonthViewRow *row = g_ptr_array_index (self->week_rows, i);
+      GcalMonthViewRow *row;
+      gboolean can_focus;
+
+      row = g_ptr_array_index (self->week_rows, i);
+      can_focus = i >= FIRST_VISIBLE_ROW_INDEX && i <= LAST_VISIBLE_ROW_INDEX;
+
+      gtk_widget_set_can_focus (GTK_WIDGET (row), can_focus);
       gcal_month_view_row_update_style_for_date (row, middle);
     }
 }