[IMP] workflow: add sequence for ordering workflow transitions (split/join mode especially for XOR)

Makes ordering transitions easier and more deterministic.

(Rebase of #1564)
[FIX] web: tooltip container set to body in all case

Mandatory since we changed the structure of the dom since 8.0.
[IMP] base/res_users: improve reading/writing of reified group fields

- simplify the code by removing unused cases
- do not modify input argument 'value' of create/write
- do not call BaseModel.read() with reified group fields (this causes warnings)