window: put the search button in the start of the headerbar

Since the search bar has been moved to above the sidelist, it
makes sense to have the search button on the start of the
headerbar, strengthening the relation between both widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=766922

shell/window.ui

@@ -176,10 +176,6 @@
               <class name="image-button"/>
             </style>
           </object>
-          <packing>
-            <property name="pack_type">end</property>
-            <property name="position">1</property>
-          </packing>
         </child>
         <child>
           <object class="GtkBox" id="top_right_box">