erp5_trade: Extract logic for further reuse

Movement_isAggregateItemListEnabled allows to define, if given movement
is a candidate for aggregation, so extract this part into
Movement_isAggregateCandidate.

bt5/erp5_trade/SkinTemplateItem/portal_skins/erp5_trade/Movement_isAggregateCandidate.py

+if context.getResource() and context.getResourceValue().getAggregatedPortalTypeList()\
+   and context.isMovement()\
+   and (('Cell' in context.getPortalType()) or not context.getVariationCategoryList()):
+  return True

bt5/erp5_trade/SkinTemplateItem/portal_skins/erp5_trade/Movement_isAggregateCandidate.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="PythonScript" module="Products.PythonScripts.PythonScript"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>Script_magic</string> </key>
+            <value> <int>3</int> </value>
+        </item>
+        <item>
+            <key> <string>_bind_names</string> </key>
+            <value>
+              <object>
+                <klass>
+                  <global name="NameAssignments" module="Shared.DC.Scripts.Bindings"/>
+                </klass>
+                <tuple/>
+                <state>
+                  <dictionary>
+                    <item>
+                        <key> <string>_asgns</string> </key>
+                        <value>
+                          <dictionary>
+                            <item>
+                                <key> <string>name_container</string> </key>
+                                <value> <string>container</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_context</string> </key>
+                                <value> <string>context</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_m_self</string> </key>
+                                <value> <string>script</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_subpath</string> </key>
+                                <value> <string>traverse_subpath</string> </value>
+                            </item>
+                          </dictionary>
+                        </value>
+                    </item>
+                  </dictionary>
+                </state>
+              </object>
+            </value>
+        </item>
+        <item>
+            <key> <string>_params</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>Movement_isAggregateCandidate</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>

bt5/erp5_trade/SkinTemplateItem/portal_skins/erp5_trade/Movement_isAggregateItemListEnabled.py

@@ -4,9 +4,7 @@ for Base_viewTradeFieldLibrary/my_view_mode_aggregate_title_list.enable
 
 # If the resource accepts items, and the context is a movements (ie. not a line 
 # already containing cells or line that has variations but not cells yet)
-if context.getResource() and context.getResourceValue().getAggregatedPortalTypeList()\
-   and context.isMovement()\
-   and (('Cell' in context.getPortalType()) or not context.getVariationCategoryList()):
+if context.Movement_isAggregateCandidate():
   return True
   
 # If the movement already has an aggregate, display it.