This is an improvement over current UI fee selection. Current UI only allows kSat/kB fees, calculated over an approximation of the tx size. Some users may want to set a specific fee in satoshis, including zero, which isn't possible. This PR adds the UI items necessary for the user to set a specific fee.
Commit comment:
Fees can now be set in satoshis instead of kSat/kB only. Fees can now be set to zero satoshis. When switching to designer fee, or when inputs or outputs are changed, tx size is properly calculated (no guesswork). Fees, specified internally in sat/B, used to suffer alterations due to those approximations.