Ability to add alternative target columns to a task later on

This issue describes the ability to add alternatives by adding files to a project later on.

• The upload feature will be selectable in task-preference window.
• The filename of the uploaded file is looked up in the file list of the task.
• The first existing file with the same name is considered as the target file.
• The content of the new uploaded file is added as alternatives to the existing file
  • if the column name already exists in the target file, the content is not overwritten
  • other columns are added
  • the alternatives are aligned by mid
  • additional segments (mid not found in target) are not imported, but sent later in a "not imported" mid list to the user
  • missing segments (mid not found in segment) remain empty, they are also sent to the user as "remained empty" mid list
• after import a short notice is shown to the user (saying if there will be a email or not)
  • if not all mids could be aligned, a email is sent to the user which mids are imported or not
  • if there are already existing column names this is also written into the email
  • if all was OK, no email is sent.
• It's not allowed to add alternatives to existing ranking tasks, because then the new alternatives will not be ranked but the old ones are.
  • if ranking has been done and the workflow is changed to another workflow still it will not be possible to add alternatives to the task (check if there are already existing rankings is sufficient)