feat: Open event routing for multi-user interfaces

[Eideren]

PR Details

UIElement.RaiseEvent is public, but events themselves cannot be created by users as the properties for those have internal setters.
This is essential for simulating multiple input methods on different UIs within the same game instance, for splitscreen multiplayer for example.

Related Issue

I think we mentioned this once but I don't remember where.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.