Transaction labels are misleading

Currently, in Specter, there is only address labelling. The data structures are:

For the CSV:
```
Address,Label
bc1qj3...,some_label
```
For the JSON:
`{"labels": {"some_label": ["bc1qj3..."], ...}`

Unlike for example in Electrum, where you have this structure:
`{"8a5e4...": "some tx id label", "bc1qj3...": "some address label"}`

The tx labels are by default currently some address that is chosen to be the somewhat relevant destinaton address for the tx:
E.g. sending to `bcrt1qhrk4jmxky43nf2jup7pz6336nr4py97td5szrp` results in:

If you change the "tx label" it will change the label for the associated address, and vice versa, which is, in particular, problematic if you have a tx where you receive, since this can lead to unexpected results:

If you label the tx with id 63882... like this:

Now, we change the label for `bcrt1q9mkrhmxcn7rslzfv6lke8859m7ntwudfjqmcx7` to:

which also changes the label for the tx itself:

Solution: Have a separate key-value pair for tx-ids and labels.
Intermediate solution: Deactivate the labels column in the tx overview.