Added VectorDataWidget setColumnEditable/getColumnEditable methods.

These allow modification of editability after construction (previously editability could only be specified using an argument to the constructor).
Improved support for CompoundVectorParameter column editability.

The user data specifying per-column editability can now be changed from a parameterChanged() callback, and will be reflected in the UI. Note that this will currently only work if the parameterChanged() callback was triggered by a parameter within the CompoundVectorParameter itself, as changes to other parameters will not trigger the _updateFromPlug() method needed to apply the changes. We may be able to improve on this in the future if we map all parameter user data into per-plug metadata entries and have the UI respond to changes in that, but that is a bigger task.
Added VectorDataWidget.selectedIndices() method.

This returns the ( column, row ) indices for the currently selected cells. Previously the internal meaning of "index" was just the row index (at some point we only allowed selection of entire rows), so I've also renamed all the old internal methods to clarify that they only refer to rows and not indices in the sense used in the public API.
Added right click preset menu for CompoundVectorParameterValueWidget.

This also adds the ability for any custom parameter menu to operate with CompoundVectorParameters, whereas before they couldn't.