[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] mail: add tooltip to send a message and log a note links
[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] website: correct image resizing
[FIX] account_voucher: validation of refund in multicurrency

Fixing several issues for refunds in multicurrency mode that prevented the moves to be balanced.
The amount_currency needs to use the absolute value, as a refund will have a negative amount.
The sign for currency_rate_difference needs to use the line instead of the voucher as they can have different value. e.g. a voucher of 1000$ with invoice of 1200$ and refund of 200$ will have two lines and their currency_rate_difference should have different signs.
Avoids doubling the value in foreign_currency_diff
Fixes #1490, opw 607118 & 611580
[FIX] l10n_be: spelling error in financial reports
[FIX] web: make search by `id` possible.

+ correct tests
[FIX] Account: Consolidation Account should not be carried to next fiscalyear
[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.
[FIX] account: payment_ids should list payments of same account than the invoice account
[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.
[FIX] website_sale_option: make options work
[FIX] account: child_of operator in invoice analysis report

When displaying invoice analysis of a company, include the invoices of the members of this company (opw 611084)
[FIX] payment_paypal: use the validate url according to the acquirer env
[FIX] payment_paypal: SUPERUSER_ID removed by inadvertence in 1826f6d4eca499754a1f4bbe7179987eab473a55