slider: handle mouse wheel events

[kiedtl]

Make slider + vertical_slider handle mouse wheel events -- i.e. when hovered, scrolling the mouse wheel will modify the slider.

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Question: currently the slider is incr/decremented by self.step for both ScrollDelta::Pixels and ScrollDelta::Lines. This seems appropriate for Lines, but I'm not sure about Pixels. (It felt fine to me when testing, but I'm not sure what the "expected" behaviour is.)

[hecrj]

Thanks!

I made a change to only enable slider scrolling when the Ctrl modifier is pressed. This way, we avoid accidental slider changes when inside a scrollable.

[kiedtl]

Oh. I was trying to emulate Windows-specific behavior here, specifically the app I was replacing with my Iced project. Of course, users can adjust, but the initial friction will be there.

So I suppose there's no way for a widget inside another to prevent the outer widget from capturing its events?