PriorityEventHandler issues with sub-item support [5.2.1]

Section order handling code, that was described in Inconsistent new item priority assignment logic [5.2.1] also has some issues with sub-item support. For example it's currently impossible to sort section images in same way (using dropdown) as sections themselves. This is because the PriorityEventHandler class, that is handling that have no knowledge of sub-items.

Solution

1. populate possible orders dropdown on new sub-item creation page (OnNew event)
2. populate possible orders dropdown when sub-item is saved to database (OnBeforeItemCreate and OnBeforeItemUpdate events)
3. add extra order position (last one), when creating new sub-item (OnNew and OnCreate events)
4. when determining a list of possible entries in orders dropdown we need to account for records in temp table (if it's being used) as well (right now only live table is taken into account)

I have patch already, that implements all but last item.

Related Tasks

INP-1404 - Getting issue details... STATUS