Add additional constructors for `UiRect` to specify values for specific fields

[Piturnah]

Objective

Often one wants to create a UiRect with a value only specifying a single field. These ways are already available, but not the most ergonomic:

UiRect::new(Val::Undefined, Val::Undefined, Val::Percent(25.0), Val::Undefined)

UiRect {
    top: Val::Percent(25.0),
    ..default()
}

Solution

Introduce 6 new constructors:

• horizontal
• vertical
• left
• right
• top
• bottom

So the above code can be written instead as:

UiRect::top(Val::Percent(25.0))

This solution is similar to the style fields margin-left, padding-top, etc. that you would see in CSS, from which bevy's UI has other inspiration. Therefore, it should still feel intuitive to users coming from CSS.

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Changelog

Added

• Additional constructors for UiRect to specify values for specific fields