Move the chart's web view to the UI definition

Fially! Thanks goes to tristan, KaL and ebassi on #gtk for the
guidance.

src/resources/ui/ag-window.ui

@@ -73,6 +73,8 @@
       <column type="gchararray"/>
     </columns>
   </object>
+  <object class="WebkitUserContentManager" id="content_manager">
+  </object>
   <template class="AgWindow" parent="GtkApplicationWindow">
     <property name="can_focus">False</property>
     <property name="has_focus">False</property>
@@ -173,6 +175,12 @@
                     <property name="pack_type">start</property>
                   </packing>
                 </child>
+                <child>
+                  <object class="WebkitWebView" id="chart_web_view">
+                    <property name="user-content-manager">content_manager</property>
+                    <signal name="context-menu" handler="ag_window_chart_context_cb"/>
+                  </object>
+                </child>
               </object>
               <packing>
                 <property name="name">chart</property>