[IMP] Sequence for workflow transition (split/join mode especially for XOR)

Sorry for the auto-formatting, but I guess it is not a big deal. Still very readable and just small functional changes.

I added the sequence to the transition b/c it was missing for the transitions (XOR) and it is a pain to do it in XML or/and via proper recreation/creation in the UI itself.

Should be possible to merge into 8.0 as it does not change anything crucial and old workflows should work like they are...at least from my point of view.

I would appreciate reviewing and recommendations and of course if this could land in v8 soon (@odony, @rco-odoo, @fpodoo, @xmo-odoo, @antonylesuisse).

I really like the workflow mechanisms although it is probably worth for more refactoring, improving the interfaces at some point and it needs to be sold properly that developers and functional architects have a common ground. It actually does give a lot of structure to a process although some people might think that it adds more complexity.

Why not use the full natural order you set on the object, i.e. 'sequence, id' ? This would have the benefit of improving also the case where all sequences are the default value, and would be more consistent.