Add a rank column to items/labels

[rathrio]

This is a prerequisite for #5.

What's the scope for item ranks? The default case is that items have one label assigned to them. So it'd make the most sense to require a unique rank within one (the first) label.

We should probably disallow reordering when items of multiple labels are displayed.

Any smarter approaches for multi-label items? Should we get rid of multiple labels all together?