Bug 1074732 - [Messages] Create a mixin to handle the select UI model

- Create a selection mixin and extend the methods into view modules.
- Unify the UI update into one method and adjust the update flow to change the selected data first, then update (visible) UI based on data.
instead of doing this, it's better to use the same mechanism than EventDispatcher:
* a `mixin` method that creates a new encapsulated instance
* calling `SelectionHandler.mixin` with potentially some arguments