[IMP] account: statement automatically set starting and ending balance

To prevent having holes between statements and for better usability using import and synchronization
we try to automatically set the starting and ending balance based on the previous existing
statement.
To do that, we added a new field previous_statement_id so that whenever we change the ending_balance
of the previous statement, we recompute the starting balance and ending balance of the current statement.

Also creating a new statement in between 2 others statements will automatically set the correct value
to the starting and ending balance of that statement and all the statements afterwards.

Exception: creating a statement by hand won't automatically set the balance_end_real, however if you
create one between 2 statements, the balance_end_real of next statements will be recomputed

Was Task #1880409

closes odoo/odoo#38696

Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>