Allow user to set default transaction speed.

**What problem are you trying to solve?**
Currently, if I want anything but the default gas price (which can result in a delay of 30 minutes or more when the network is busy) I have to manually change it.

I would like to be able to specify that I want my transactions to complete in a certain amount of time by default. The gas price should be computed from my desired transaction time and the current gas market.

**Describe the solution you'd like**
A switch that changes between entering gas price and entering desired transaction time. The amount you enter in either field retains its value for future transactions.

Start by tracing where the extension currently lets users set gas prices and where transaction settings are retained. Compare that flow with the requested switch between gas price and desired transaction time, then identify how current gas-market data could support the calculation. Done means the selected mode and value persist and transactions use the resulting default setting.

Written by the indexing model from the issue text.