By listening only to the relevant expandedChanged and rowHeightChanged
signals we avoid accessing invalid data on clearing. When clearing the
models each one is collapsed. The collapsing in turn triggers
rebuilding of the labels. Unrelated labels could access already
cleared data like this.

Change-Id: I3fad45f5c6279019fbc27ec54e758f498a9d55f2
Reviewed-by: Kai Koehne <kai.koehne@digia.com>