Layout APIs should be leading/trailing instead of left/right

[porglezomp]

For better UI localizability, alignment, padding, etc. APIs should use "leading" and "trailing" instead of "left" and "right". This way, you can design widgets which work automatically with both left-to-right and right-to-left text directions.

• As precedent in UI, Apple's SwiftUI defines alignment with .leading and .trailing options.
• As a precedent in Rust, the str::trim_left method was deprecated and replaced with str::trim_start.

This would still leave the larger task of making widgets handle different text direction locales, but it would be nice if the names in the API didn't get in the way of approaching this.

Places where this seems definitely appropriate to replace with leading/trailing, just looking very briefly:

• iced::alignment::Horizontal
• iced::Padding

I'm not sure about iced::widget::tooltip::Position, but I think it's appropriate to use leading/trailing here.

I think iced::widget::pane_grid::Direction is probably appropriate remaining left/right, since as I understand it it's used for navigating the visual representation of the layout.