Issue #3173943: Make target selection same height as source selection in mapping form

added 3 commits

added 11 commits

04c346c5...605f7137 - 5 commits from branch project:8.x-3.x
c77f4c98 - Made a start with using an after build callback for a mapping row per property.
32e19727 - Bottom border for mapping rows in any admin theme
c9ea3d4c - Added mapping row classes and css.
5c7f508c - Fixed some test failures.
3d5f68dd - Added some code comments.
37b58f39 - Adjusted CSS for some themes.

merged

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