Add value field to Enum Literal

The "value" field is an extension to the UML 2.5 metamodel. It manages
the internal representation of enumeration value in system programming
languages.

Add value field to new Enum Literal properties dialog, and class
UMLEnumLiteral, and widget Enum. Add new attribute "value" to
"UML:EnumerationLiteral" element in xmi file format, and increase
the version of XMI_FILE_VERSION.

BUG:358091
FIXED-IN:2.18.80 (KDE Applications 16.03.80)
REVIEW:126612