virtual events are not displayed in time window not containing their root element

Repeated on last trunk runbot:

 http://trunk-14493.runbot.openerp.com

This bug is probably present since 2010, and cripples agenda recurrent event.

To repeat:
1 - Go to Messaging / Calendar ...
2 - Click somewhere in the calendar to get the create event popup.
2 - choose a name for the event, then go in the Tab "Options", click on "recurrent"
3 - choose "repeat every:" 1 week, "Until" Number of repeatition: 52, check "monday"
4 - Save

5 - Now try to load/reload the calendar view in the months after your current date and that do not contain the starting date.

I expected to see the next events since I have asked for a recurency every week on monday.
What I see is that my recurrent event are not displayed.

Diagnosis
-------------

All events (virtual and real) are correctly shown in tree view. But not if you create an advanced filter to filter dates
as the calendar view will do.

Recurrent events are not in database but are created thanks to ``base_calendar`` modules and generates ``virtual ids`` for virtual events that stems out the recurrence rule of the first event.

The ``create``, ``write``, ``search`` rules are thus overwriden to catch any request on the database and try to give life to these virtual ids.

The ``search`` algorithm is quite bugged and seems to be half implemented.

I'm posting a branch to correct this behavior (and a bunch of other) in a short while.