Usability improvements on account_credit_control:

* Generation of lines:
 * Display the report on the main page (prevents user to have to switch page to view the result)
 * Add a button that opens the generated lines
 * Move the button in the <header>
* On the invoice:
    * Put the policy field in a <group>
    * Hide the policy field when empty (irrelevant if empty)
    * Use a correct label for the policy field
    * Remove useless fields from the tree view for the lines: it was
      overflowing from the sheet view
* Add the date of the day on the report
* Move the 'run' menu before the 'lines' menu