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.
In the new try, I left only checkInputs on view module. and maybe we could even move checkInputs to selection if we want to do it aggressively(ideally we can have some smarter way that put delete(and make in the future) element/method as options for edit UI init) And maybe even fetch the elements only inside the selection module instead in view module and pass as parameter since they are not used in view module.