Rework Balance Sheet and Profit and Loss so that they are more consistent with other reports:
 - Ability to choose output format and produce in deferred mode
 - Allow specifying the date range explicitly, for current period and previous period as well. When dates are not specified, the previous behavior of guessing start date and previous date range from accounting periods is kept
 - Style fixes
 - Set a filename for *Save as ...*

Introduce a new feature on Profit and Loss to allow to have instead of the two column layout (used in France mostly):

| INCOME  | EXPENSE |
|---------|---------|
| **1. Revenue** | **1. Operating Expenses** | 
|     - ...... | - ...... | 
|    | - *Cost of Sales* | 
|     | - *Selling, General and Administrative Expenses* | 
|     | - *Other* | 
| **2. Non Operating Income** | **2. Non Operating Expense** | 
|     - ...... | - ...... | 
| **3. Interest Income** | **3. Interest Expense** |
|     - ...... | - ...... | 

Have a one column layout with intermediate profit/loss calculated for each level ( what I call "british layout" here):

| Profit and Loss  |
|---------|
| **1. Operating** |
|  * *Sales* | 
|  * *Operating Expenses* | 
|        - ...... |
|        - Cost of Sales | 
|        - Selling, General and Administrative Expenses | 
|        - Other | 
| **2. Non Operating** |
|  * *Non Operating Income* | 
|        - ...... | 
|  * *Non Operating Expense* |
|        - ...... | 
| 3. **Financial** |
|  * *Interest  Income* |
|        - ...... | 
|  * *Interest Expense* |
|        - ...... | 

This is achieved by putting all that hierarchy in `portal_categories/financial_section/income` and not splitting it in `income` and `expense`. This report will not display `expense` category if it is empty.

Still TODO:
* [x] updating https://www.erp5.com/erp5-HowTo.Select.Accounting.Financial.Section

/reviewed-on nexedi/erp5!284