Fix nuxeo-selection-toolbar default z-index

Since ELEMENTS-662 (10.2), adding a nuxeo-date-picker in a selection action's dialog, when you open it, it is displayed behind the dialog overlay as shown in attached screenshot.

This is caused by the nuxeo-selection-toolbar default z-index value 201 while the inner vaadin-date-picker 's overlay z-index is 200.

Setting 199 for nuxeo-selection-toolbar fixes the issue.

The workaround is to define:

--nuxeo-selection-toolbar: {
  z-index: 199;
}

in your theme since this mixin is applied to nuxeo-selection-toolbar.