[ADD] account_reversal_usability

[sbidoul]

I propose the creation of a module that improves the usability of account move reversal:

• add a to_be_reversed flag on account.move (we had that in the past in the account_reversal OCA module)
• this flag is set to False when a reversal is done (best way to do it to be investigated, it could be on write of the reversed_entry_id field of the reversing entry)
• add a "To Be Reversed" search filter (we had that in the past in the account_reversal OCA module)
• show the reversal_move_id field in the view as Reversal Entry (it is a o2m, but in practice it is a one2one, and the computed m2o to be displayed could be called reversal_id for compatibility with the old OCA module)

We could then improve some OCA modules that generate entries to be reversed to set this flag (for instance, in OCA/account-closing).