[1.6.1] Being able to remove a category without deleting all transactions

If I want to remove a category that I already used for some transactions, those transactions would currently also be removed.

It would be better if we have a choice here:

1. remove all transactions for the category of the category gets removed (but I don‘t know when I would need such a usecase)

2. keep the transactions and just remove the category that is selected inside the transaction. That the transaction at the end is saved without any category selected.

3. Select an alternative category with which the current selected category (that I want to delete) is being replaced with. Migrating the transactions from the category I am about to delete to another existing category.