file chooser: Avoid tall combos

The filter combo box in the file chooser was stretching to match
the height of the extra widgets next to it. Since, stretched combo
boxes are not a pretty sight, make it stick to the top instead.

Patch by Caolán McNamara,

https://bugzilla.gnome.org/show_bug.cgi?id=751227

gtk/ui/gtkfilechooserwidget.ui

@@ -346,6 +346,7 @@
                 <property name="focus_on_click">False</property>
                 <property name="entry_text_column">0</property>
                 <property name="id_column">1</property>
+                <property name="valign">start</property>
                 <signal name="changed" handler="filter_combo_changed" swapped="no"/>
               </object>
               <packing>