Implement rounding options for "Pay-By-Mail" payment type.

[wp07e]

#1014

Adds a combo box with rounding options to the nearest integers 1, 5, 10 or 20.

Tested on 1.0.7 seems to be working fine.

All other relevant totals (i.e. Amount of XMR to sell, Min amount of XMR) are also updated as well.

There is one slight issue where the new combo box seems to re-render out-of-order so I will prob. submit another commit to resolve that but its not a functional issue.