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

- Unbind the selected inputs logic from DOM tree structure. Create a selection module that listens to input select changed and return the selected data.
ideally, I'd like to have a mixin instead of an object. The idea is that this functionality would "blend" in the object.
It does not make a huge difference but I think it makes more sense for this.

what do you think?