[layout] Allow `orderAttribute` to accept duplicate and non-consecutive values

[dsinni]

As it is now, it seems that when setting orderAttribute, the attribute values must be unique, consecutive and must also start from 0 or items are not displayed.

Regarding starting from 0, bind-for in state makes using this impossible, due to #245

It would be great if duplicate values and gaps in values were also allowed as strictly consecutive, unique numbers starting from 0 are not always practical.

See this demo for examples: https://glitch.com/edit/#!/silicon-cold

Thanks!