Fix KAboutData::applicationData() to init from current Q*Application metadata

Also
* warn if there is no Q*App instance yet to set properties in KAboutData::setApplicationData
* check and warn if KAboutData::applicationData() is out-of-sync with qApp data
* remove bogus check for empty display name, as the method defaults to
  componentname
* unit tests KAboutDataApplicationDataTest::testInteractionWithQApplicationData

REVIEW: 127655

Thanks mpyne and aacid for review