Additional check when concatenating paths

This suggestion came up in the discussion about !119 (merged):

• @rgugel started a discussion: (+1 comment)

  this check could also be useful in TQFolder::concatPaths (so we might want to move it there). Small addition though: include the passed path segments for easier tracing of issues

In TQFolder::concatPaths: check if the second path starts with "/". Warn user that this might be an absolute path.