Change the default for the measure_func field of ContentSize to None.

[ickshonpe]

Objective

The default for ContentSize should have the measure_func field set to None, instead of a fixed size of zero. This means that until a measure func is set the size of the UI node will be determined by its Style constraints. This is preferable as it allows users to specify the space the Node should take up in the layout while waiting for content to load.

Solution

Derive Default for ContentSize.

The PR also adds a fixed_size helper function to make it a bit easier to access the old behaviour.

Changelog

• Derived Default for ContentSize
• Added a fixed_size helper function to ContentSize that creates a new ContentSize with a MeasureFunc that always returns the same value, regardless of layout constraints.

Migration Guide

The default for ContentSize now sets its measure_func to None, instead of a fixed size measure that returns Vec2::ZERO.
The helper function fixed_size can be called with ContentSize::fixed_size(Vec2::ZERO) to get the previous behaviour.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

@ickshonpe ping on the migration guide, but I'll merge this tomorrow regardless.

[ickshonpe]

Migration guide fixed and added a little helper function. Should be all ready.

[alice-i-cecile]

The helper function is a great idea :)