Add a layer for layout application

[Hermanest]

The problem

Currently layout is designed the way that controllers are responsible for layout application. This approach allows the system to be simple, but disables a lot of capabilities.

The solution

Shift the responsibility by making ILayoutItem accept the dimensions instead of exposing the entire RectTransform instance.
Current variant:

public interface ILayoutItem : ILayoutRecalculationSource {
    //...

    RectTransform BeginApply();
    void EndApply();
}

Replace with:

struct LayoutDimensions {
    public float TopOffset;
    public float LeftOffset;
    public float Width;
    public float Height;
}

public interface ILayoutItem : ILayoutRecalculationSource {
    //...

    void Apply(LayoutDimensions dimensions);
}

This approach enables us to implement an intermediate layer for animations, allowing lerping between states without knowing the exact values.