The onboarding information is currently initialized for the primary
editor stack in the shared area. When the shared area is split into
multiple part stack showing editors side-by-side and closing the last
editor in the original part stack, the onboarding information controls
get lost together with that part stack.

With this change, onboarding information is embedded into every part
stack of the onboarding area, such that when closing one of them, the
information does not get lost. To this end, every part stack is in the
shared area is tagged as an editor stack and the onboarding information
is added to every tab folder of part stacks tagged as an editor stack.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1773