Allow disabling the rounding of payment amounts

[pittybitty]

Currently, the amounts to be received are automatically rounded by haveno to improve user privacy. Unfortunately, the rounding of the amounts means that small trade amounts cannot be traded effectively as the price gaps are very large due to the rounding. This discourages smaller traders from using the software, as trading smaller amounts is not really worthwhile.

For this reason, I suggest that the rounding of the amounts to be received is optional and can be switched on and off using a switch or an option. This would also allow smaller trading amounts to be traded sensibly and, above all, more precisely.

[haveno-user]

Also make it easier to set rounding to amounts ending in 5, 10 or 20, so pay-by-mail traders don't have to stock 1s.

[wp07e]

@haveno-user has a good point and this is definitely needed. I will take a stab at it.