Add a filter form on top of default content views listing document content

The content view used to display document listings does not define any search document model.
If it was defined, it could be used as a filter form on top of the listing, to add more criteria and reduce the number of results.

The filter form should be displayed in a drop down box, closed by default.
When opened, it displays the search form and two buttons "submit" and "reset".
When submitted, the box should be closed and a message displayed on it saying "Filter is activated". When "reset" button is hit, or when no filter has been previously submitted, this message should not be displayed.