Group the keyframe selection parameters together depending on the method
(regular or smart) they belong to.
Parameters for a method are only shown if that method is the currently
selected one.
This commit removes most of the existing parameters for the keyframe
selection and adds new ones to enable the regular keyframe selection.
Since the regular method is the only one available for now, the parameter
that allows the user to enable it or not is disabled.
Set the value of the string parameters in the nodes' description as
the unevaluated variable instead of the evaluated variable. The
evaluation will be made later, when the nodes with such parameters
will be computed.
