Let umbrello generate UML 1.4 compatible xmi format on writing enum literals.

UML 1.4 spec mentions the following xml tag hierachy:

UML:Enumeration
   UML:Enumeration.literal
      UML:EnumerationLiteral
      UML:EnumerationLiteral
      ...

umbrello wrote:

UML:Enumeration
   UML:EnumerationLiteral
   UML:EnumerationLiteral
   ...

Loading of enum literals also supports the mentioned hierachy.