Support cross-storage merging operation

[jinlan]

Currently only multiple stack inside the same storage support merging stack operation.

[lilwhitemouse]

Behavior is equal to vanilla shelves (and multiple vanilla storage zones). If the behavior is changed in vanilla or by mod that affects vanilla, Deep Storage will automatically inherit changes.

One possibility of doing it all in Deep Storage would be to have Storage Units share a single SlotGroup (the storage zone). The technical challenges are high: if settings are changed for one Storage Unit, it would need to remove itself from the other SlotGroup and create its own with different storage settings. It would also have to look for another Storage Unit that shares its new settings, and if it finds one, expand that one's SlotGroup, etc.

Not simple.