Make Retraction sub-settings indented to improve visual usability

[Sophist-UK]

Description

This PR simply makes the Retraction sub-settings indented so that it is more readable and the location of these settings is more easily identified.

This is the seventh of several PRs to do this for other groups of settings.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I have made this same update in my installation of Cura 5.6.0 and tested that the changes work.

Test Configuration:

• Windows 10
• Cura 5.6.0
• File: C:\Program Files\UltiMaker Cura 5.6.0\share\cura\resources\definitions\fdmprinter.def.json

Checklist:

• My code follows the style guidelines of this project as described in UltiMaker Meta and Cura QML best practices
• I have read the Contribution guide
• I have commented my code, particularly in hard-to-understand areas
• I have uploaded any files required to test this change

Additional Comments

The Retract Before Outer Wall setting remains visible even when Retraction is disabled and despite the setting definition have the exact same enabled property as the Retract at Layer Change setting which isn't shown. I haven't managed to track this down, but I suspect that the same setting is being defined elsewhere without or with a different enabled property.