Eliminate accessible children-changed event flood on container repopulation

When the canvas container is repopulated (e.g. when using back/forward, or pressing Esc while in a search), finish_adding_new_icons is called leading to an accessible-children-changed event flood. We can distinguish these unwanted notifications from desired notifications by comparing the number of new icons to add with the size of the container's icon set.

Note that we cannot reliably identify this condition by monitoring the is-loading property of the view: Loading officially can end while icons are still being added. This is especially true in the case of very large icon sets, which is the exact scenario we're trying to avoid.