feat(scroll-tabs): add container overflow class / preinitialized scrollers
feat(scroll-tabs): add container overflow class / preinitialized scrollers

This adds essentially two features:

* a library may provide pre-initialized scroll buttons
(will be created, if not existinging)
* the overflow class is attached to the container rather
than the scroll buttons. This enables users to do advanced
semantic styling based on the overlay status.

BREAKING CHANGE:

* `scroll-tabs-button-hidden` cls got replaced with
`scroll-tabs-overflow` on the container.

bpmn-io/scroll-tabs