Display correct balance if payment is pending

[manuelwedler]

follow up from #14

Is your feature request related to a problem? Please describe. When submitting the payment dialog, it would be helpful for the user if the sent amount is subtracted from the visible channel balance.

The problem with this is that when submitting the payment dialog, it is not clear which channel will be used for the payment. For local routing several routes could be tried until the payment completes. The pending_transfers endpoint could provide the currently tried route.

If in the first place the sent amount is subtracted from a different channel than the channel which is used for the successful payment, we have a bad UX. That is why I am not sure if this feature makes sense at all.