Isolate Active Layer & Group are now separate states.

As requested by Bollebib and Dmitri, the isolation active layer and
group modes are now separate, independent states with inherent
priority.

When Isolate Active Group is enabled, only the siblings of the
isolation root (usually the active node) are visible.

When Isolated Active Layer is enabled, only the isolation root (usually
the active node) is visible.

Finally, when both Isolate Active Group and Isolate Active Layer are
enabled, Isolate Active Layer takes priority and only the isolation root
(again, usually the active node) is visible.