qt: provide an option to use Kirigami's wheel handler in flickables

Unlike our handler, Kirigami's handler can do smooth scroll. Since Qt's default behavior is also smooth scroll (no custom handler), I have not bothered supporting smooth scroll in our custom scroll handler.

In fact, the reason we came up with a custom scroll handler was to provide an option to disable smooth scrolling (!659 (merged)). Unfortunately Qt does not provide an option to disable smooth scrolling.

Currently there is no intention to remove our custom handler, or make the new kirigami handler the default. Kirigami's handler seems to not behave well with nested flickables when touchpad is used. At the same time, our handler causes the content to jump which seems to be started happening after the Qt 6 update, that I can reproduce with touchpad. It is not clear if this is a Qt bug or a bug in our handler.

The best would be Qt to fix scrolling and provide an option to disable smooth scrolling so that we do not bother with custom handlers. Although improved, Qt's scrolling is still buggy after many years. I think that Kirigami's handler can be especially useful for people who want to keep smooth scrolling on, and don't want to rely on the default scrolling behavior of Qt.

Request review @chub.