qt: obey the predefined scroll direction in `FlickableScrollHandler`

This class is used for scrolling, it should obey the predefined scroll direction.

Currently on platforms invertedness is provided (and when the platform plugin supplies the information), this causes the scrolling direction to be always non-inverted (which means that natural scrolling is not used even if it is wanted).

I have tested on Wayland.

Request review @chub.