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.