In number fields, Up/Down and a wheel notch added a whole step and kept
the fraction, so 5.1 mm stepped up to 6.1 mm. A plain step now goes to
the next whole unit in its direction (5.1 goes up to 6 and down to 5; 5
goes up to 6 and down to 4), and a Ctrl/Cmd step goes to the next tenth,
in the field's own unit. Repeated presses or notches of one size add or
take one step each, and presses and notches are applied in the order
they came. A Shift step still adds or takes ten and keeps the fraction.
The number fields that widgets.rs draws for the panels, the Control bar
and the dialogs (unit, percent, degree and count fields) all use this
step (widgets::step_value).