Make the source domains unmovable.

At the moment, when mapping domains, you can choose different options for both the source and the target.

I propose we always just pre-load the source and make it so the source drop down boxes cannot be changed (or perhaps turn them into text blocks).
This will help the user avoid setting one source domain to two target domains (which is invalid) and will fix a bug I am having where the source drop down boxes seem to change value while scrolling through the list. It will also help ensure the user has mapped all source domains, which is needed on fields that cross types (string -> int).

This is a bit like issue #203 with the caveat that the pre-loaded source values shouldn't be movable.