Make use of QFile for reading / writing xml files / hashes.

This should fix some issues when hashfiles contain newline characters.

- Also the xml file is read only once
- XMLQtInterface is now responsible for reading and validating the file
  (see XMLQtInterface::readFile())
- Added QByteArray XMLQtInterface::_fileData for caching the file
Added a comment.
Merge pull request #259 from bilke/Refactor-XMLQtInterface

Refactor XMLQtInterface