[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
Merge remote-tracking branch 'upstream/8.0' into 8.0
[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] web: make search by `id` possible.

+ correct tests
Merge remote-tracking branch 'upstream/8.0' into 8.0
[FIX] orm: _compute_related: ensure prefetch is done in batch

Due to the use of a sudo env, the records
were being added to the sudo cache one by
one instead of all at once. This meant the
prefetching was not able to load all
records at once, leading to prohibitive
times when processing thousands of
records.
Merge remote-tracking branch 'upstream/8.0' into 8.0