list: Allow storing size in the ListTile

list: Allow storing size in the ListTile

... and use it to handle ListView allocations.

Nothing spectacular, just proof of concept.

The code introduces the idea that every tile stores its area (others
would call it "allocation", but I avoided that because tiles aren't
widgets). This should allow moving lots of code into gtklistbase.c and
not require special handling inside ListView and GridView.

And that in turn hopefully makes it easier to add more features (like
sections and so on.)