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.
some thoughts:
* you can use `for (var item of this.selectionHandler.selected.values()) { ... }`
* you can keep the "selectedList" and do: `var delNumList = items.map(item => +item);`