Updates `NavigationItemImpl` constructor to accept a `Supplier` which
supplies a list of children instead of passing the list its self in
the constructor.

This allows the work of constructing a navigation sub-tree to be
deferred and only completed if the `NavigationItem#getChildren()`
method is called.

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refs adobe#2526

Updates the V1 and V2 `NavigationImpl` models to pass a `Supplier` to
their respective `NavigationItemImpl` constructors.

Removes `Navigation#getExportedType()` because it is exactly the
same as the super method.

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refs adobe#2526

Updates the breadcrumb model implementations by removing the `children` field
from `BreadcrumbItemImpl` constructor because it is always set to empty list (i.e.
breadcrumb items never have children).

The `BreadcrumbItemImpl` will now pass a `Supplier` which
supplies an empty list to the super class `NavigationItemImpl`.

Removes `BreadcrumbImpl#getExportedType()` because it is exactly the same
as the super method.

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refs adobe#2526

…tructors

Updates the V1 and V2 `LanguageNavigationImpl` models to pass
a `Supplier` to their respective `LanguageNavigationItemImpl`
constructors

Fixes a possible NPE in `LanguageNavigationImpl` if
navigation root is not set in either properties or
policy.

Removes `LanguageNavigationImpl#getExportedType()` because it
is exactly the same as the method from the super class.

Update how titles are determined for language navigation items to
reuse the logic used for all other navigation items.

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refs adobe#2526