Grid layout's add transition should allow animating things other than x,y

The "move" and "add" Grid layout (and Row and Column) transitions only allow
x and y to be animated. This is fine for the "move" event, but one could use more for the
"add" event (designers are unlikely to want the new item to merely slide in from the top
left corner). Thus, animating the scale and opacity (at least) for use in "add"
transitions would be nice to have supported too.

The same goes for remove transitions