Make capturing events for HdySwipeTracker implicit

This is the first part of !505 (merged).

The reason it was explicit is because you can't pass user data to a capture event handler, and so only have access to the widget in question, and to the event, and HdySwipeable doesn't expose its HdySwipeTracker either.

This makes it use g_object_set_data() to avoid this, and have HdySwipeTracker register its own handler.