collection-view: Keep track of hidden items
collection-view: Keep track of hidden items

We don't immediately add new items to the view before UI is out of
wizard state since commit 94414dc but we don't keep track of these
hidden items, which results in these items being add to the view even if
user cancel's their creation and items don't actually exist. This
results in ghost VMs being added to the view.

This patches fixes the issue by add a list of such hidden items and
ensuring they don't get added if user changes their mind.

https://bugzilla.gnome.org/show_bug.cgi?id=749725

vbenes/gnome-boxes,vbenes/gnome-boxes