[FIX] Exporting of res.partner works again

Singleton object was required while access model properties, but search returns multiple results and hence caused traceback while accessing record.property
[FIX] website_livechat: reference of demo data was preventing to install the module

Added a record of im_livechat.channel in website_livechat data
[FIX] Allow search on `_inherits` fields.

As `_inherits` fields are now handled via `related`
fields (not stored, obviously), a new descriptor
`searchable` has been added to `fields_get()` result
to indicated if the field is searchable or not.
[IMP] fields: add mechanism to extend a selection field

If a selection field is defined by a list as selection, such as:

    state = fields.Selection([('a', 'A'), ('b', 'B')])

one can extend it by inheritance by redefining the field, as:

    state = fields.Selection(selection_add=[('c', 'C')])

The result is that the selection field will have the list
[('a', 'A'), ('b', 'B'), ('c', 'C')] as selection.
[IMP] models: name_search() should call _name_search()

This avoids code duplication between methods, and keeps backward compatibility
with existing code overriding _name_search().
[FIX] project_issue, resource: fixed computation of hours / days to open (assign) or close an issue. Those fields now have a conditional store, because they were not recomputed even when changing date_open and date_closed. Changing the stage now correctly changes the date_closed. Also added some fixes in resource module, the date was not correctly taken into account when computing the number of working hours between two dates.
[FIX] crm: fixed day_open and day_close computation (conditional store + be sure that date_open and date_closed fields are set)
[FIX] mail: compose wizard: fixed default_get overrides not correctly managing the received field_list + improved custom call to default_get to correctly manage its in and out values.