Added flicked-Callback on Flickable

[pfzetto]

Hello,
as requested in #1462, I've added a scrolled-Callback that gets triggered when the viewport_x- or viewport_y -Properties are changed by a mouse drag or mouse wheel movement.

I was only able to test this change on the Rust API (by adding a debug statement to one of the examples), because my current Nix build setup isn't setup for C++, Python or NodeJs.

Thanks,
pfzetto

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[pfzetto]

Thinking about it,flicked may be a better name?

[ogoffart]

Thanks a lot for the pull request.

As for the name, i think both scrolled and flicked are acceptable. Also not sure which one is better.

I noticed that you do not emit it in mouse move, is that intentional?

It should be documented in https://github.com/slint-ui/slint/blob/master/docs/reference/src/language/builtins/elements.md#flickable

Please also make a test.
For example adding something in https://github.com/slint-ui/slint/blob/master/tests/cases/elements/flickable.slint that does scrolled => { root.scrolled-called += 1; } and check it is called when it should. (see https://github.com/slint-ui/slint/blob/master/docs/testing.md#rust-driver for how to run the test with a filter)

[pfzetto]

Hello,
I've previously didn't emit on mouse move because I thought of the Event to be triggered after the drag operation.
Thinking about it further, I added it because I think that it would feel weird to users when the content is only loaded after they finished dragging.
I've also decided to rename the callback to allow for separate dragged and scrolled callbacks in the future.

Thanks,
pfzetto