Widgets.Attachment.Box: Hide if empty

data/ui/widgets/status.ui

@@ -259,7 +259,6 @@
                     </child>
                     <child>
                       <object class="TootleWidgetsAttachmentBox" id="attachments">
-                        <property name="visible">True</property>
                       </object>
                       <packing>
                         <property name="expand">False</property>

src/Widgets/Attachment/Box.vala

@@ -40,6 +40,7 @@ public class Tootle.Widgets.Attachment.Box : FlowBox {
     public bool pack (API.Attachment obj) {
         var w = new Widgets.Attachment.Slot (obj);
         insert (w, -1);
+        show ();
 
         return true;
     }