[IMP] ir.qweb: avoid repeatedly searching the registry for nonexistant widget models
[IMP] calendar: simplify/optimize `rrule` computed field

Avoid reading one record at a time, leading to multiple
calls to read() which is special-cased and even slower
than a normal one.
[FIX] calendar.read: do not re-compute `display_time` for virtual calendar ID if not requested
[IMP] calendar.event: _compute should be done in batch + consistent @multi attributes
[IMP] orm.Field: doc for @copy attribute
[IMP] hr,mail,product,project,base: use @auto_join=True on _inherits FK field

This can give a performance boost on large databases
and should not be a concern in terms of access control
as the inheritance already grants access to the parent
records.
[IMP] fields.Date[Time]: convert_to_cache: avoid costly parsing when not validating

The extra parsing check is not necessary when we're
not validating inputs, because in that case the
values come from the database and are valid.
The validation is quite expensive due to calls
to strptime() + strftime().