MappingEditor: introduce option to only create 1-to-1 mappings

[nichtich]

A toggle switch with visual indicator to enable/disable default mapping cardinality. At the moment it is hard-coded to 1-no-n. At least 1-to-1 should be selectable to avoid adding a second concept at the target side if doing 1-to-1 mappings.

1-to-1

1-to-n

n-to-1

n-to-m