Rework "Apply / Discard" dialog for switching apps

First, create this dialog on demand instead of having it always exist, and move out of delegate to be at the top level component's scope. Then, fix width, so that it fills all the available view now. Also, add application name & icon to it, so user knows what is the app in question. And finally, adapt it to narrow mode by turning row into column dynamically using GridLayout with columns: narrow ? 1 : 2