fix ATK's "showing" event emission

"showing" events were only bound to map/unmap events, but sometimes the first map event was already handled when the ATK object was connected to it which leads to strange behaviours like never emitting the True signal but later the corresponding False. Found this because it prevents Orca from speaking tooltips (no True signal). Fix behaviour by hooking also into "visible", using the same check as for state representation (as documentation says) and emitting a signal when this actually toggles the state.