[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 strptime() calls when passed strings