refactor(shell): retire the Jobs destination

Nothing it carried is gone -- the two commits before this put all of it
somewhere the work is already being done.

A retired destination is the one shape #25's storage decision does not
make free. A visibility entry is a map key and an unknown key is
dropped on load; a launch page is a *value*, and an unknown one fails
validation -- which on the load path means the app refuses to start for
whoever had Jobs selected. `RetiredViews` is that list, read by
`ApplyDefaults`, which treats a retired name as a zero value. An
unknown-but-not-retired name still errors, because that is a typo and
saying so is the useful answer.

backend/config/views.go

@@ -24,10 +24,25 @@ const (
 	ViewExplore   View = "explore"
 	ViewDownloads View = "downloads"
 	ViewAutotag   View = "autotag"
-	ViewJobs      View = "jobs"
 	ViewSettings  View = "settings"
 )
 
+// RetiredViews are destinations that used to exist and no longer do.
+//
+// A *visibility* entry for a removed view needs no such list: it is a
+// key in a map, and an unknown key is dropped on load. A `DefaultPage`
+// is a **value**, and an unknown one fails validation -- which on the
+// load path means the app refuses to start rather than a setting being
+// ignored. So the one shape that cannot be retired for free is named
+// here and reset to the default instead.
+//
+// `jobs` was folded into Settings by #27: library scans under
+// Libraries, index work under Search Index, downloads under the
+// download clients, and the autotag apply into the Autotag view.
+var RetiredViews = map[View]struct{}{
+	"jobs": {},
+}
+
 // ViewSpec is what the backend knows about a destination. The label and
 // the icon are deliberately absent: those are presentation, they live
 // beside the rest of the app's icon vocabulary in
@@ -73,7 +88,6 @@ var Views = []ViewSpec{
 	{ID: ViewExplore, VisibleByDefault: true, Hideable: true, CanLaunch: true},
 	{ID: ViewDownloads, VisibleByDefault: true, Hideable: true, CanLaunch: true},
 	{ID: ViewAutotag, VisibleByDefault: false, Hideable: true, CanLaunch: true},
-	{ID: ViewJobs, VisibleByDefault: true, Hideable: true, CanLaunch: true},
 	{ID: ViewSettings, VisibleByDefault: true, Hideable: false, CanLaunch: false},
 }