The functionality for creating and identifying editor stacks based on
the "EditorStack" tag is scattered across several code places. This
change centralizes the according functionality in the recently
introduced PartStackUtil. This utility class currently contains
functionality related
to the primary data stack, which also relies on being an editor
stack.

In addition, this change reverts the preliminary fix for visible names
of open editors being to short (#1712) as this became obsolete with the
recent improvements of handling primary data stacks and editor stacks
(#1775, #1778).

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/1570
Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/1773