Add extensible Scene Editor controls

[ABKQPO]

Summary

This pr introduces an extensible Scene Editor control api for toolbar buttons and dropdown menu items

• Adds registries and reversible registration handles for contributed toolbar buttons and Export/Snap menu items
• Exposes an action context, icon model, embedded menu-widget contract, and reusable slider widget for integrations
• Moves built in Scene Editor controls into a dedicated default control registry while preserving existing Export and Snap behavior
• Adds an optional “Open Structure Folder After SNBT Export” preference
• Fixes mouse wheel routing so interactive scenes in the Guide Editor preview receive wheel input before the preview scrolls

Checklist

• I have tested this PR in DevEnv
• I have tested this PR in Fullpack
• This PR is in compliance with the GTNH AI Policy
• This PR requires another PR in order to merge