Allow draging accounts in Preferences into new scenes

Tusker/Screens/Preferences/PreferencesView.swift

@@ -36,6 +36,9 @@ struct PreferencesView: View {
                                         .foregroundColor(.secondary)
                                 }
                             }
+                        }.onDrag {
+                            let activity = UserActivityManager.mainSceneActivity(accountID: account.id)
+                            return NSItemProvider(object: activity)
                         }
                     }.onDelete { (indices: IndexSet) in
                         var indices = indices