Rework the icon view grid sizing and placement code a bit

It's still a little messy, but this cleans things up a little.  The
problem is that we could get into a situation where we're trying to
place items but we have no grid yet.  So now I'm simplifying things a
bit, and separating loading the items from the model from actually
placing them on the grid.  We also just protect actions that need a grid
with a check to see if we have a grid, since just because the widget is
realized, it doesn't mean it has a grid.

In addition, the previous code was using the widget's size request to
size the grid (and falling back to its preferred size otherwise).  This
is bonkers: we can only size the grid based on the widget's actual
allocation; anything else can give us sizes that are wrong.