[VarDumper] implemented expanding of all children

Fixes: #12612

It was not possible to expand all children of a big collection. This made the VarDumper component unpracticle, if you
want to get an overview on a deeply nested datastructure.

This PR shows a second (double) arrow infront of the single arrow. Clicking on it expands the whole subtree.


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12612
| License       | MIT

Please revert this change: it is not required for the feature implementation and makes reading the diff harder