Acsone addons distribution list improvements

**Improvements of `distribution_list`**

* Rename 'Source Model' into 'Model' for `distribution_list_line`
* Found a native solution for custom CSS (hide elements into create/edit mode)
* Hide `see result filter` into edit/create mode to avoid unwnated saves for user
* Reset `domain` when model change into `distribution_list_line`
* Add a `widget selection` for `many2one` on `ir.model` (more safe)
* Fix tests with `notify_email` because of this partner selection's field now contains only two values
* Can now save an expression by a dynamic way depending the `ir.model` which is present on the `distribution_list_line` 
This is a string for a filter element so it is required :)