Add methods for calculating the size and postion of UI nodes

[ickshonpe]

Objective

Users are often confused about how to calculate the size and position of UI nodes.

Solution

Implement methods on Node to calculate its size and position given its GlobalTransform.

Changelog

• Added methods logical_rect and physical_rect that calculate the geometry of a UI node and return it as a bevy_math::Rect

[alice-i-cecile]

This will be super useful for tests too.

[ickshonpe]

For some reason it pulled in my changes from the UI example fix PR. Fixed, I think.