Bug 962434 - Moving value selector into subcomponents of an appWindow. I...

...mproving its visibility a11y.
The lifecycle of the value_selector sub-component is handled by the appWindow: the sub-component is destroyed when the window is being destroyed, and when that happens app.element is deleted from the document thus getting read of these even listeners. The same happens with the value selectors own listeners that are initialized in the _registerEvents method when the value selector is destroyed it's element node is deleted from the document. 
AFAIK, we do not need an explicit removal of listeners here (similar to app_modal_dialog for example).