iconGrid: Change IconGrid.addItem() to take an object instead of an actor

IconGrid has never really been a general purpose container, but has
always been used in conjunction with BaseIcon. IconGrid will soon
gain the ability to adjust the item size dynamically to adapt to the
available space, which will require that we can make some more
assumptions about the items added to the grid (namely: we need
access to BaseIcon's setIconSize() method).
So change addItem() to take an object instead, which should have
an actor and a (BaseIcon) icon property.

Based on a patch by Carlos Soriano.

https://bugzilla.gnome.org/show_bug.cgi?id=706081