systemtray HiddenItemsView: allow tab focus and set currentIndex

If activeFocus is true and currentIndex is not set, set currentIndex to
0.
If activeFocus is false and currentIndex is set, unset currentIndex.

applets/systemtray/package/contents/ui/HiddenItemsView.qml

@@ -54,5 +54,12 @@ PlasmaComponents3.ScrollView {
             }
         }
         delegate: ItemLoader {}
+
+        activeFocusOnTab: true
+        onActiveFocusChanged: if (activeFocus && currentIndex === -1) {
+            currentIndex = 0
+        } else if (!activeFocus && currentIndex >= 0) {
+            currentIndex = -1
+        }
     }
 }