Fix writing of output files on export to DocBook/XHTML:

umbrello/docgenerators/docbook2xhtmlgeneratorjob.cpp
- In function run(), after call to xsltSaveResultToFd() close the
  tmpXhtml file.

umbrello/docgenerators/docbookgeneratorjob.cpp
- In function run(),
  - after call to umlDoc->saveToXMI(file) close file;
  - after call to xsltSaveResultToFd() close the tmpDocBook file.

umbrello/umlmodel/umlrole.cpp
- In function saveToXMI address error about redefined attribute reported
  by xmlParseFile() called from {Docbook,Docbook2Xhtml}GeneratorJob :
  Remove writing of attribute "visibility", it is already written by
  UMLObject::save1().

umbrello/version.h
- Increment XMI1_FILE_VERSION to 1.7.6 and XMI2_FILE_VERSION to 2.0.3
  due to changed generation of AssociationEnd attribute "visibility".

CCBUG: 449661