API CHANGE: SetExpression VariableName is read-only

The implementation in Writer has been read-only since initial CVS
import, so adapt com.sun.star.text.textfield.SetExpression to
implementation.

Change-Id: I70cdd5d0a44cb86426f533c00f86874e8dde8b87
SERVICE: /UCR/com/sun/star/text/textfield/SetExpression
 incompatible change: Field 11 ('VariableName') of r1 is not longer a
 property of this SERVICE in r2
 incompatible change: Field 11 ('VariableName') of r2 is a new
 property compared to this SERVICE in r1 and is not 'optional'

offapi/com/sun/star/text/textfield/SetExpression.idl

@@ -74,7 +74,7 @@ published service SetExpression
     [property]double Value;
     /** contains the name of the set expression field master this field is connected to.
      */
-    [property]string VariableName;
+    [property, readonly] string VariableName;
     /** determines whether changes in language attributes at the position the text field
         is located also change the number format as appropriate for this language.