Store "radius" parameter of Circle layer as ValueBase.
A few TODO notes
Store "angle" parameter of Star layer as ValueBase.
Update UnSetStatic action too.
Reorganize save routines for static option
Add interpolation option for ValueBase. Implement loading and saving for it.
Reorganize code. Now interpolation option isn't lost when parameter is changed.
Describe ValueBase::copy_properties_of()
Display the assigned interpolation in the parameter type column (for non-animated items only)
Show icons for interpolation types in waypoint context menu.
Add new action - ValueDescSetInterpolation.

At the moment works for non-animated layer parameters only.
Add submenu to set layer parameter interpolation using the right-click.
Allow Cairo render to use Value Base as parameter on Circle Layer
Fix bug: Change interpolation doesn't work for exported parameters.
When casting a value node it is needed to check if the returned value node handle is valid because you can have a value node that is not the type you casted to. For example, a exported value node is first returned as layer parameter but from the dynamic parameters. See value_desc.cpp
Only show Interpolation contextual menu for layer parameters, layer parameter exported constant and exported value node constant.
Later when we teach the animated value nodes or other value nodes to change its interpolation, more conditions should be shown here.
Re indent code and add member to know if the value description is animated value node.
Move the get/set members for interpolation property to Value Node class. Add custom interface for derived classes that can implement it.
Simplify checking of given value description to allow to show the 'Set Interpolation' contextual menu
Simplify code to perform or undo the ValueDescSetInterpolation action based on if the value description is a value node or not.
Add code to save and load interpolation attribute for animated value nodes.