Added support for fixed size CompoundVectorParameterValueWidget.

The ["UI"]["sizeEditable"] user data entry can be given a BoolData value of False, which will cause the +/- buttons to be hidden in the UI, enforcing a fixed length on the data in the vector parameters.
Added per-column editability to VectorDataWidget.
Added per-column editability to CompoundVectorParameterValueWidget.

This uses a ["UI"]["editable"] user data entry in each child parameter, where a BoolData value of False will make the column for that parameter read-only. I would have liked to have used the existing ["gaffer"]["readOnly"] entry which controls the editability of the underlying plug values instead, but that would prevent resizing of the data when only some columns are editable.

Fixes #766.
Merge pull request #816 from johnhaddon/compoundVectorParameter

CompoundVectorParameter UI improvements