assistant: Add margin to action area

We are no longer setting a margin on the content area, so put
one on the action area itself to prevent the buttons from touching
the edge.

https://bugzilla.gnome.org/show_bug.cgi?id=762315

gtk/ui/gtkassistant.ui

@@ -51,6 +51,7 @@
                 <property name="visible">1</property>
                 <property name="halign">end</property>
                 <property name="spacing">6</property>
+                <property name="margin">6</property>
                 <child>
                   <object class="GtkButton" id="apply">
                     <property name="label" translatable="yes">_Apply</property>