QVT — variable-assignment isn't defined in Core language

There exist assignments to variables in the QVT examples.

However, variable-assignment is not defined in the abstract and concrete syntax of the Core language.

This should be fixed.

Impact: only changes the QVT Core language definition.

(1) In Section 9.17.8 (Assignment), replace all the introductory text
by an "An assignment sets the property of target object or sets a
the value of a variable. See PropertyAssignment and VariableAssignment
definitions for detailed semantic of property assignment and variable
assignment."
Also, remove the 'targetProperty' association description.
(2) In Section 9.17 add a new class named "PropertyAssignment" with
the introductory description copy pasted from the actual description
of Assignment class.
Add the following sub-titles after the introductory text:
"""
Superclasses
Assignment
Associations
targetProperty: Property [1]
The property whose value is to be assigned.
The target property should be defined on the type of the slot expression.
"""
(3) In Section 9.17 add a new class named "VariableAssignment" with
the following definition:
"""
An assignment sets the value of a variable.
Superclasses
Assignment
Associations
targetVariable: Variable [1]
The variable whose value is to be assigned.
"""
(4) In Figure, 9.3 make the needed changes so that the diagram reflects
(1), (2) and (3) actions.