Added DatagridMapperTest, ListMapperTest. Improved ShowMapperTest.

This PR improves code coverage of lines to 36.12%
But it also brings another possible BC: new methods are added to the AdminInterface and 1 method is added to the DatagridInterface.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -