Partly eliminate the use of Property.toString().

Partly eliminate the use of Property.toString().

The toString() method should not be used on a property to get its value.
Added getStringValue() to AbstractField, Label etc. Using them
where applicable. Added comments to some problematic locations where
Property.toString() is known to be used.

AbstractField.toString() and Label.toString() still return the same
values as before, but it will throw an exception in future revisions.

Migration needed: Replace explicit and implicit uses of
Property.toString() with use property.getValue() and its
string representation.
Alternatively, use AbstractProperty.getStringValue() and
AbstractField.getStringValue() instead of Property.toString() during
migration.