Fix installer.dat not found issue

 * use applicationDirPath() instead of applicationFilePath()

Change-Id: I2f25dd88d3c024b20d0ef1937e7a8e50a8183594
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Merge remote-tracking branch 'origin/1.3' into 1.4

Change-Id: I9fd96d5906048e6d258490f82d93c7f2ef942b09