Split transfer

[mtotschnig]

Since transfers can now be categorized, it should also be possible to create "split transfers". We already allow to create split transfer parts, but currently those are linked to "stand-alone" transfer peers.
This ticket is about allowing split transfer parts, that are linked to other split transfer parts.
Suggested implementation: When a split transaction is saved, and it contains several transfer parts linked to the same account, on the transfer account, a split transaction is created, that groups together all corresponding transfer peers.

[mtotschnig]

Related: #681

[JPP91]

Ce serait vraiment un plus d'avoir cette possibilité de transfert ventilé.
Possibilité que j'utiliserai fréquemment lors de mes transferts, afin de pouvoir vérifier à quelle catégories sont affectés les différents montants et indispensables pour faire un pointage correct avec les relevés de vanques