Partially revert previous commit. Do not use ValueBase for canvas parameter.

If implemented, further warning messages (Canvases still open!) are raised when opening synfig-core/examples/logo.sif and close it. ValueBase for canvas parameter doesn't add any functionality so it is not implemented.
Do not use EXPORT macro for canvas parameter since it will be removed in the future.
Do not offer the parameter interpolation sub menu if the parameter is canvas type