UnitsApi::setSchema() was being called when the user changed the
comboBox item, so the view system was saved if the user later clicked
Cancel. This call was moved to the saveSettings() method that is called
only if the user clicks Apply or OK.
GUI settings now computed directly from this value. This eliminates the
possibility of the combobox index and the value it represents getting
out of sync.
Load/save operation complete. Not yet integrated with the display code.
Also would like to disable option when non supported (those not using
fractional inches) are selected.
