Polish DnD Examples.

- Remove class DragLabel in draggabletext and add a static creation
  function instead since it only has a constructor setting some
  properties.
- Use QRegularExpression instead of QRegExp
- Use new connection syntax.
- Ensure compilation with
  DEFINES += QT_NO_CAST_TO_ASCII QT_NO_CAST_FROM_ASCII
  demonstrating use of QLatin1String vs QStringLiteral.
- Streamline code.

Change-Id: I2e2ddeb40837dba379990836c77fb72ad7263e2d
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>