Fix the Size helper functions using the wrong default value and improve the UI examples

[ickshonpe]

Objective

Size::width sets the height field to Val::DEFAULT which is Val::Undefined, but the default for Size is Val::Auto.
Size::height has the same problem, but with the width field.

The UI examples specify numeric values in many places where they could either be elided or replaced by composition of the flex enum properties.

related: #6498, #7468

Solution

Change Size::width so it sets height to Val::AUTO and change Size::height so it sets width to Val::AUTO.

Changelog

• Changed Size::width so it sets the height to Val::AUTO
• Changed Size::height so it sets the width to Val::AUTO.
• Added tests to geometry.rs for Size and UiRect to ensure correct behaviour.
• Simplified the UI examples. Replaced explicit values with the flex property enums where possible, elided values where possible, and removed the remaining use of auto margins.