Rename STRING_VALUE@value attribute to displayText

[mvdzel]

There are multiple meanings in the CIMI RM to (in this case) the attribute named "value". One is in ELEMENT@value where the value is a DATA_VALUE_TYPE and one is in the STRING_VALUE where it is defined as "The displayable rendition of the text item, regardless of its underlying structure. Defined as unicode text.".
My proposal would be to make sure that we check the whole model for these issues.
And rename the STRING_VALUE@value to @displayText to better reflect what it is for.