Expander header's height determination

[sohaibbg]

The height of Expander's header is now determined based on the height of its children.

Pre-launch Checklist

• [x] I have updated CHANGELOG.md with my changes
• [x] I have run "dart format ." on the project
• [x] I have added/updated relevant documentation

[sohaibbg]

This was achieved by encasing the content widget in a Flexible widget so the Column can size the header as per its children's requirements.

[bdlukaa]

Expander padding is off

[sohaibbg]

Expander padding is off

In latest commit, padding has been added to Container widget responsible for header.

[bdlukaa]

The trailing is being constrained down, for some reason