AbstractItem: add activated signal. An input type agnostic way to activate systray items was needed. This signal will be used for keyboard, mouse and touch navigation. There was an activated() function that just started an animation which
has been renamed to startActivatedAnimation().
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.
HiddenItemsView: allow Return, Enter and Space to activate items