hotkey to follow spending outputs / prevouts

[chappjc]

It would be helpful to have hotkeys for navigating to spending transactions, or back to the funding transactions that created previous outputs.

For instance if there is a transaction that creates two outputs, e.g. (1) 12.345 DCR, and (2) 200 DCR. A hotkey like ] should follow the link to the transaction that spends (2) if it is spent. Also hitting number keys could navigate based on that corresponding output index, unless this would get hairy to implement.

Similar navigation can be done for the inputs and their previous outpoints.