Added per-column visibility for CompoundVectorParameterValueWidget.

Visibility is specified using the standard ["UI"]["visible"] userData entries on the child parameters which provide the columns.

Also added setColumnVisible/getColumnVisible methods to VectorDataWidget to support this.

Fixes #526.
Merge pull request #599 from johnhaddon/compoundVectorParameterColumnVisibility

Added per-column visibility for CompoundVectorParameterValueWidget.