Derived classes cannot use `Tabbable (_content_vbox,..)`
`_content_vbox` is a member of Tabbable (which has not
yet been initialized) at the point of construction.
This breaks internal API, hence the omnibus commit
Actions for hide/show/attach/detach tabbables use hard-coded names which are not
translated. Using Tabbable/WindowProxy::name() to lookup the action will fail,
since the name can be translated. This changes just removes the option to not
provide a menu-name when creating these objects, and uses the name menu-name
when looking up an action by name
