To snap an icon to the closest grid cell, we were trying to round
the x position with an offset of 0.5. However, cells are not
normalized, so that 0.5 is actually doing little to round the
position.

This is making the icons to snap to the right closest cell once the
user drags more the icon left to the current cell.

The actual size of the cell is SNAP_SIZE_*, so to fix this use half
of the SNAP_SIZE to "round" the number.

https://bugzilla.gnome.org/show_bug.cgi?id=750446