property_sheets: generate value accessors for source/destination accounts on default supply

Without these accessors, we have to use constructs like:

    resource.edit(
        default_purchase_supply_line_destination_account='account_module/123'
    )

with the accessors, we can use:

    resource.edit(
        default_purchase_supply_line_destination_account_value=account,
    )

The former is a bit error prone, because typos in the property name
silently create a local propery and typos in the relative URL make a
"broken" relation.