[BUGFIX] fixed some bugs in the datagrid

__Tasks:__
- [x] removed rows are removed from selected
- [x] selecting all emits the correct number of selected items
- [x] got feedback

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none