Convert items when they are added to an actor

[Ethaks]

Converting items Metron did not convert yet (as per the item's flags) when they are added to an actor might be worth looking into.
A check for that could be done in the preCreate hook, where the conversion functions should be able to generate a suitable update object to apply to the item's source.

Notes:

• maybe restrict this to compendium items
• check whether conversion functions can work with preCreate source (should be okay, as the source contains full item data unlike creation data)