iconview: layout items immediately when setting a GtkTreeModel

iconview: layout items immediately when setting a GtkTreeModel

As the draw handler expects the items to be laid out already, we cannot
queue a layout here to avoid a race condition with the resize that is
queued immediately after, which in turn would lead to a segfault later
in the paint_item() implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=663138