Replace Length with impl Into<Length> in width/height APIs

[TitouanReal]

Widgets in iced allow to change their height and their width with an API like this :

pub fn width(mut self, width: impl Into<Length>) -> Self

This is especially useful when setting a fixed width/height:

my_widget.width(30);

instead of

my_widget.width(Length::Fixed(30.));

This PR proposes to change the API to make it more in line with the one in iced.

[genusistimelord]

thank you, this was still set to the old way of doing things.