Funnels: add sorting by steps order

Currently funnels return results in unspecified order. Clients generally prefer to show results in accordance with steps order defined in the data schema. This is not currently possible without workarounds.

It would be nice to have a way to define sorting as 'steps ordering' and may be 'steps backwards ordering'. Funnels are most useful with natural steps ordering so this option should probably be enabled by default.

If step names alphabetical order corresponds to their logical order then it is possible to order them by name achieving the same effect. Usually this is not the case so step names should be changed accordingly which leads to additional processing on client to map step names to something more user friendly.