plugin app manager: various improvements: - Generated password must now be named and are stored, so they are re-used on following restarts. Password size can now be specified. - New `not` filter for `!libervia_param` to inverse a boolean value. - Former `front_url` field has been renamed to `web_url_path` as it is the URL path used for web frontend. All Web frontend related field are prefixed with `web_`. - `front_url` is now used to specify a whole front URL (notably useful if an app uses its own domain). A list can be used to retrieve a key, like for `url_prefix`, and `https` scheme is added if no scheme is specified. - An abstract class is now used for App Managers. - Last application start time is stored in persistent data.