main-window,new-call-box: Move origin store from MainWindow to NewCallBox

The main window has no need to store origins now so this simplifies
things dramatically.

src/ui/new-call-box.ui

@@ -3,6 +3,14 @@
 <interface>
   <requires lib="gtk+" version="3.22"/>
   <requires lib="libhandy" version="0.0"/>
+  <object class="GtkListStore" id="origin_store">
+    <columns>
+      <!-- column-name name -->
+      <column type="gchararray"/>
+      <!-- column-name origin -->
+      <column type="GObject"/>
+    </columns>
+  </object>
   <template class="CallsNewCallBox" parent="GtkBox">
     <property name="can_focus">False</property>
     <property name="halign">center</property>
@@ -16,6 +24,7 @@
     <child>
       <object class="GtkComboBox" id="origin_box">
         <property name="can_focus">False</property>
+        <property name="model">origin_store</property>
         <property name="id_column">0</property>
         <property name="margin_bottom">8</property>
         <property name="no_show_all">True</property>