Added DatagridTest

Improved code coverage of lines to: 37.46% (36.12% before)

This PR introduces another small BC break - new methods are added to FilterInterface and PagerInterface.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -