Stop a room's lastActiveTime from changing because of hidden events

This was just a missed oversight during the refactoring, we need to pass
the hidden filter into NeoChatRoom::lastEvent so it doesn't pick up on
hidden events and push rooms to the top for no discernible reason.

Also, I simplified the function to take out the cached event handling,
because lastEvent is already doing that. The function is const now, too
and has some nicer comments.

src/libneochat/neochatroom.h

@@ -208,7 +208,7 @@ public:
     bool visible() const;
     void setVisible(bool visible);
 
-    [[nodiscard]] QDateTime lastActiveTime();
+    [[nodiscard]] QDateTime lastActiveTime() const;
 
     /**
      * @brief Get the last interesting event.