Coarse sliders

This is a proposed patch to implement #9939.  I've only added coarse steps to a handfull of sliders, but many more are candidates; each one requires an additional function call during slider initialization to set the coarse step size and changing the accelerator registration call from dt_accel_register_slider_iop to dt_accel_register_slider_iop_coarse.  I tried auto-detecting whether the slider has coarse steps defined, but the widget doesn't yet exist when dt_accel_register_slider_iop is called.

The large steps sizes are currently hard-coded as 1/3 EV, 10%, and 100 Kelvin, but they should be made into user preferences.