If an icon already exists in an app view with the same id, the
duplicate is not added on a call to addItem. Unfortunately,
since it's not added, the icon actor gets orphaned and leaked.
This commit address the problem by introducing a new hasItem
method and disallowing callers to call addItem with a duplicate
in the first place.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/628
eaa32090b9