added tab_menu template to use dropdowns in tab menu

replaces the pull request https://github.com/sonata-project/SonataAdminBundle/pull/2069

- there is no place where i could possibly a test in (as far as i know)
- tests are still working
- a usage example is shown below

```
$menu->addChild('appointments', ['label' => 'Termine', 'attributes' => ['dropdown' => true]]);
```