history-box: Use content style for listbox

So as to match the design and less code to maintain

src/calls-history-box.c

@@ -78,25 +78,6 @@ update (CallsHistoryBox *self)
 }
 
 
-static void
-header_cb (GtkListBoxRow   *row,
-           GtkListBoxRow   *before,
-           CallsHistoryBox *self)
-{
-  if (!before)
-    {
-      return;
-    }
-
-  if (!gtk_list_box_row_get_header (row))
-    {
-      GtkWidget *header =
-        gtk_separator_new (GTK_ORIENTATION_HORIZONTAL);
-
-      gtk_list_box_row_set_header (row, header);
-    }
-}
-
 static void
 delete_call_cb (CallsCallRecord    *record,
                 CallsHistoryBox    *self)
@@ -178,11 +159,6 @@ constructed (GObject *object)
     (self->model, "items-changed", G_CALLBACK (update), self);
   g_assert (self->model_changed_handler_id != 0);
 
-  gtk_list_box_set_header_func (self->history,
-                                (GtkListBoxUpdateHeaderFunc)header_cb,
-                                self,
-                                NULL);
-
   gtk_list_box_bind_model (self->history,
                            self->model,
                            (GtkListBoxCreateWidgetFunc)create_row_cb,

src/ui/history-box.ui

@@ -53,7 +53,7 @@
                 <property name="margin">12</property>
                 <property name="valign">start</property>
                 <style>
-                  <class name="frame"/>
+                  <class name="content"/>
                 </style>
               </object>
             </child>