kcm: Fix root not being found due to loss of a context in deleted delegate

ListView's delegates are not reliable, because they may be destroyed
(for various reasons, not only because a device was removed, e.g. they
may get recycled because another device got connected and shifted the
list downward, causing some delegates to go out of view). Ideally the
code would bind root object directly by value instead of capturing it
in a local javascript closure, but AFAIK such level of memory control
(early binding) is not available to QML/JS.