• introduce a config variable coin_chooser_output_rounding that enables/disables output value rounding by the coinchooser
• by default it is disabled
• expose control of config variable to qt gui
• also; changed how the rounding works if there is only one change output
  • the rounding logic seems to have been written with multiple change outputs in mind; where it makes sense to allow a change output to have more-or-less precision than other outputs
  • with only one change output, the ideal outcome should be to have a change output that doesn't have the most precision; with this change we aim for as much precision as the otherwise most precise output

closes #3255