Refactor offer/trade related classes in core and desktop

[ghubstan]

These refactoring changes are for reducing existing and potential duplication coming with the addition of new trading protocol support in the gRPC API. Some minor styling and logic simplification changes are also included.

• Convert OfferUtil to injected singleton, and move various offer related utility methods into it.

• Delete both MakerFeeProvider classes, which were wrappers around the same static old OfferUtil method.

• Inject OfferUtil into CreateOfferDataModel, CreateOfferViewModel, TakeOfferDataModel, TakeOfferViewModel, MutableOfferDataModel, MutableOfferViewModel, OfferDataModel, EditOfferDataModel, EditOfferViewModel

• Refactor TakeOfferViewModel

  Use OfferUtil, remove unused fields & methods.
  Made minor logic simplification, style and formatting changes.

• MutableOfferDataModel

  Made minor logic simplification, style and formatting changes.

• MutableOfferView uses new paymentAccount.isHalCashAccount().

• MutableOfferViewModel

  Refactored to use new VolumeUtil, CoinUtil, OfferUtil.
  Removed unused fields & accessors.
  Made minor style change.

• Refactored OfferDataModel to use new OfferUtil

• Refactor CreateOfferService

  Inject and use OfferUtil
  Move some utility methods to OfferUtil
  Remove unused fields

• Offer

  Refactored to use new VolumeUtil for volume calculations.
  Made stateProperty and errorMessageProperty fields private.

• PaymentAccount

  Moved isHalCashAccount type check to this class.
  Moved getTradeCurrency logic to this class.

• Contract, TradeStatistics2, TradeStatistics3

  Refactored to use new VolumeUtil for volume calculations.

• Trade

  Refactored to use new VolumeUtil for volume calculations.
  Made minor logic simplification, style and formatting changes.

• CoinUtil

  Moved some coin utility methods into this class

• CoinUtilTest

  Moved (coin related) tests from CoinCryptoUtilsTest and OfferUtilTest
  into CoinUtilTest, and deleted OfferUtilTest, CoinCryptoUtilsTest.

• Adjust create and edit offer tests to model refactoring

[sqrrm]

One minor comment. I think it's otherwise good.

To make it easier to review, it helps to keep refactorings separated in different commits. In particular pure refactor work and minor improvements are very helpful when kept separate.

[sqrrm]

utACK