Resizable: Callbacks ignore the grid

If a resizable has a grid defined, the 'resize' callback continuously fires as though the box were resizing normally, one pixel at a time. The result is an erroneous and largely useless callback, essentially representing only user mouse movement.

Also from what I can tell the same logic is causing the ui.size.width argument that's passed to the 'stop' callback to be erroneous, having the value the box would have had if it were not constrained to a grid, instead of its true final width.