Add the tree level as css class to each row

[JanisGawrychCGI]

What kind of change does this PR introduce? (check one with "x")

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior? Added datatable-row-level-i to the classes of the body row, with i representing the level in the tree structure, to allow for further css styling.

Does this PR introduce a breaking change? (check one with "x")

• [ ] Yes
• [x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information: