If a DataNamePropertyWidget is created for a DataNameProperty whose
AccessInfo == READ, it doesn't instantiate a QLineEdit to display the
property's value (QComboBox is used instead).

Previously, the updateWidgetFromProperty method didn't check AccessInfo
to see what display widget was in use, and simply always dereferenced
the _lineEdit pointer. If AccessInfo == READ, it was null and the
application crashed.

Now updateWidgetFromProperty accesses only the combo box if
AccessInfo == READ, and uses the line edit otherwise.

Fixes #99