Groupings cannot be overwritten

[dlebech]

It seems that if a transaction is already in a group, it cannot be re-assigned to a new group. This is probably good, but it is inconvenient if an existing group is being amended with a new transaction.

Let's say we have Transaction A, B, C, D. A and B are already grouped.

• Grouping A, B and C: allowed, because it's just adding an extra transaction
• Grouping A and D: not allowed because it would put a transaction in two groups.