Use synchronizedSet

[PayDevD]

For issue #1071

GlowDoubleChest extends GlowSuperInventory and its field, parents' GlowInventory viewers are unmodifiable.

protected void initialize(List<GlowInventory> parents, InventoryHolder owner,
        InventoryType type, String title) {
        SuperList<GlowInventorySlot> slots = new SuperList<>();
        SuperSet<HumanEntity> viewers = new SuperSet<>();

        for (GlowInventory parent : parents) {
            slots.getParents().add(parent.getSlots());
            viewers.getParents().add(parent.getViewersSet());
        }

        initialize(slots, viewers, owner, type, title);
        this.parents = parents;
    }

However, viewers should be modifiable because openInventory logic modifies its viewers.
UnmodifiableSet could occur UnsupportedOperationException.
We can use synchronizedSet instead of unmodifiableSet.

[claassistantio]

All committers have signed the CLA.

[mastercoms]

Thank you for your contribution to Glowstone! LGTM!

[PayDevD]

@mastercoms
I would appreciate if you could merge this branch.
Thanks for your review!