erp5_trade: rework `isQuantityEnabled` and `isQuantityEditable` scripts

The two scripts were a bit too independant from one another, sometimes causing
weird behaviours. Editing a quantity now depends on the quantity being enabled,
which happens when the line is not expected to have cells or sublines.

Also enforces expected behaviour for items: if a product or service has
*required* items, its quantity cannot be edited, since quantity of the items
should set the quantity of the line or cell.

bt5/erp5_trade/SkinTemplateItem/portal_skins/erp5_trade/Movement_isQuantityEditable.py

-"""This script is used to know if quantity can be edited by user.
-
-* If this is not a movement (line containing lines or cell), user
-cannot edit this line which is just a container, but no actual movement.
+"""
+This script is used to know if quantity can be edited by user.
 
-* If this line has variation category list, then it means it's a line that
-will contain cell, so it's already not possible to set quantity, user have
-to create cells and set quantities on cells.
+* If quantity is not enabled on the movement, quantity cannot be set.
 
-* If items are used, quantity is set by the item quantity.
+* If items are required, quantity is set by the item quantity.
 """
 
-if not context.isMovement():
-  return False
-
-if context.getVariationCategoryList(omit_optional_variation=1) and not 'Cell' in context.getPortalType():
+if not context.Movement_isQuantityEnabled():
   return False
 
-return not (context.getResource() and context.getResourceValue().getAggregatedPortalTypeList())
+return not (context.getResource() and context.getResourceValue().getRequiredAggregatedPortalTypeList())

bt5/erp5_trade/SkinTemplateItem/portal_skins/erp5_trade/Movement_isQuantityEnabled.py

-return context.isMovement() and not context.getVariationCategoryList()
+"""
+This script is used to know if quantity should be shown to the user. Usually, not showing quantity happens when a total quantity is shown instead, meaning the line uses cells.
+
+* If this is not a movement (line containing lines or cell), user cannot see quantity for this line which is just a container, but no actual movement.
+
+* If this line has required variation category list, then it means it's a line that will contain cell, so total quantity will be used instead.
+
+* If this line has optional variation category list, and already contain cells, total quantity should also be used.
+"""
+
+if not context.isMovement():
+  return False
+
+if not 'Cell' in context.getPortalType() and \
+    (context.getVariationCategoryList(omit_optional_variation=1) or \
+    context.hasCellContent(base_id='movement')):
+  return False
+
+return True