[Feature Request] undelegate specific amount

[rachyandco]

When undelegating, the user can choose to undelegate all or any inputted amount between 0 and the fully delegated amount.

Where does the feature fit in the Nym real estate?

• Wallet

What is this solving?
This helps rebalancing delegations between mixnodes when, for example, a mixnode reaches more than 100% saturation. Right now a token holder can only undelegate all the tokens then redelegate to the same node risking to loose its place.

Also token holder might want to remove partial delegation of several mixnode to delegate to a new mixnode.