Disabled VectorDataWidget remove button when selection is empty.
Selected newly added rows in VectorDataWidget.

Also grabbed keyboard focus so the new row can be edited immediately.

Fixes #204.
Fixed double update in VectorDataPlugValueWidgets.

When setting the plug value after rows were added, they weren't disabling the _updateFromPlug() call that would be triggered. This meant they would call VectorDataWidget.setData(), wiping out the carefully added selection,
Fixed VectorDataWidget button highlighting bug.