ENH: Add ctkSpinBox DecimalsStyle::UseShortcut

The ctkSpinBox::DecimalsStyle controls how the decimals are handled. In the
regulard (Fixed) mode, the spinbox behaves just like a QDoubleSpinBox, i.e.
the number of decimals is set by the setDecimals method.
In UseShortcut methods, the ctkSpinBox responds to keyboard shortcuts to
add (CTRL + and CTRL =) decimals, remove (CTL -) decimals and restore the
default number of decimals (CTRL backspace) set by the setDecimals method.

This led to the update of the ctk widgets that use the ctkSpinBox. For most
of them it's pretty straightforward as they can listen to the
decimalsChanged(int) event and update themselves.
In the case of the slider widget, the AutoSpinBoxWidth was extend to
support the change of width due to a change in decimals as well as the
change in decimals (that is propagated to the siblings).

See Issue #2480