Propagate value proxy to other widgets

--WIP--

Adding the value proxy to ctkDoubleSlider, ctkSliderWidget and
ctkCoordinatesWidget.
For ctkDoubleSlider, the proxy is added much like it was for the double
spinbox.
For the ctkSliderWidget, since it's using the ctkDoubleSlider and the
ctkDoubleSpinBox, we can simply pass the value proxy to these. The same
thing applies to ctkCoordinatesWidget.

The tests added are very similar to the ctkDoubleSpinBox test, just tweaked
to these classes specific API.