For example, if property_sheet "prop" was moved from BT 1 to BT 2 and BT 1 doesn't exist anymore.

Before this commit we have this order:
1.  install BT 2 ("prop" exist and isn't touched)
2. remove BT 1 ("prop" is removed)
=> at the end "prop" doesn't exist anymore

After this commit we have this order:
1. remove BT 1 ("prop" is removed)
2. install BT 2 ("prop" is readded)
=> at the end "prop" exists and is OK

/reviewed-on nexedi/erp5!866