addContainer(int index, DockContainer)` ignores the index for the model list, desyncing it from `getItems()

[dprutean]

DockContainerBranch.addContainer(int index, DockContainer container) inserts the region at index
but appends the container to the model list:

childContainers.add(container);              // appended, index ignored
container.setParentContainer(this);
getItems().add(index, container.asRegion()); // inserted at index

Any call with index != childContainers.size() therefore leaves getChildContainers() and
getItems() in different orders. Every method that looks a child up in one list and writes to the
other by that index then addresses the wrong child — replaceContainer, setContainerSizePx0,
setContainerResizable and setContainerCollapsed all do exactly that.

Reproduction

No stage or scene needed (JavaFX toolkit started only because the containers are Nodes):

Bento bento = new Bento();
DockBuilding building = bento.dockBuilding();

DockContainerRootBranch root = building.root("root");
root.setOrientation(Orientation.HORIZONTAL);

DockContainerBranch center = building.branch("center");
root.addContainer(center);

DockContainerBranch sidebar = building.branch("sidebar");
root.addContainer(0, sidebar);          // intent: sidebar left of center

// model  = [center, sidebar]
// visual = [sidebar, center]           // <-- already out of order

DockContainerLeaf a = building.leaf("a");
DockContainerLeaf c = building.leaf("c");
sidebar.addContainers(a, c);
sidebar.removeContainer(c);             // sidebar prunes to one child

The prune calls root.replaceContainer(sidebar, a), which takes i = childContainers.indexOf(sidebar) = 1
and then does getItems().set(1, a.asRegion()) — but index 1 of getItems() is center.

Observed after the prune:

model  = [center, a]
visual = [sidebar, a]

center and everything docked inside it is silently detached from the scene graph, while sidebar
stays on screen despite no longer being in the model. In our application that surfaced as the entire
editor area disappearing, and a user could trigger it by dragging alone.

Expected

getChildContainers() and getItems() stay in the same order, so addContainer(0, x) puts x
first in both.

Suggested fix

childContainers.add(index, container);

Version

0.16.0, and the code is unchanged on master as of today.