Get the levinsthein distance of a post-edited segment from its MT input

Calculate the distance between MT input and the different editing stages in the workflow - Save this distance in its on task column in LEK_task - Show and visualize it in the same way as outlined for the editing time.

Main effort is the visualization. The estimated effort is based on the assumption, that "Eavluate postediting time" is implemented - the visualization can be reused from there. Therefore the development time for the visualization is not part of this estimation.