GtkFileChooserDialog: Avoid a bottom border in the dialog

We recently started to respect border-width on the action_area
again, so set its border-width to 0 so it doesn't get in the way.

https://bugzilla.gnome.org/show_bug.cgi?id=722211

gtk/resources/ui/gtkfilechooserdialog.ui

@@ -16,7 +16,7 @@
         <property name="spacing">0</property>
         <child internal-child="action_area">
           <object class="GtkButtonBox" id="dialog-action_area1">
-            <property name="border_width">6</property>
+            <property name="border_width">0</property>
             <property name="can_focus">False</property>
             <property name="layout_style">end</property>
             <child>