file chooser dialog: Prevent uneven button heights

The height of the text buttons depends on the font height,
whereas the search button has a fixed-size icon in it...
Prevent unevent heights by putting them all in a size group.

https://bugzilla.gnome.org/show_bug.cgi?id=745263

gtk/ui/gtkfilechooserdialog.ui

@@ -53,4 +53,7 @@
       </object>
     </child>
   </template>
+  <object class="GtkSizeGroup" id="buttons">
+    <property name="mode">vertical</property>
+  </object>
 </interface>