Remove "Choose automatically" option from drop-down, that chooses source or target for connecting the layout with

This drop-down decides, whether the layout is connected with the target or the source column.

So far the option "Choose automatically" connected it to the target, if at least some segments had translations. This leads to a lot of misunderstandings on wrong selections.

Therefore we remove the "Choose automatically" option and set "source" as new default.

In the API we still support "Choose automatically", but raise a deprecated message.