[FIX] export: add the context in the export

In some cases, the data dispalyed in a list depends on the context
This context needs to be passed to the export method, so the exported data reflects correctly the data from the list view
[FIX] context_timestamp MUST return a "timezone aware" timestamp
[IMP] calculating nextcall datetime of a cron SHOULD use the cron's user
[FIX] base: revert 19e401a for title on res.partner form

The title of the partner is restricted with a context that will change in the onchange_type. Having twice the same field will not work well.
Remove the invisible attribute as the domain is handled by the domain.