Fixed VectorDataWidget bool delegate for PySide.

PySide seems to have a few deficiencies in this area compared to PyQt - see comments for details.
Improved VectorDataWidget numeric editing.

We now use a GafferUI.NumericWidget for editing rather than a QWhateverItWas. This provides a more consistent experience with the rest of Gaffer and fixes some graphical problems.

Fixes #637.