[Gtk] Implement RadioButton

[JunielKatarn]

Description of Change

Adds missing implementation of RadioButton for Gtk.

Issues Resolved

None

API Changes

Added:

• class Xamarin.Forms.Platform.GTK.Controls.RadioButton
• Xamarin.Forms.Platform.GTK.Renderers.RadioButtonRenderer

Platforms Affected

• Gtk

Behavioral/Visual Changes

RadioButtons under Gtk get rendered, shown, and behave consistently with other platforms.

Before/After Screenshots

Not applicable

Testing Procedure

1. Start ControlGallery.Gtk.
2. Open the RadioButtonGroup page.
3. Verify RadioButtons show up and grouping behavior works.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jsuarezruiz]

Thanks for the contribution @JunielKatarn!

[JunielKatarn]

Thanks for the contribution @JunielKatarn!

My pleasure!
Anything else needed to merge this change?

[JunielKatarn]

Hey @jsuarezruiz. Is this change good to go?

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Technically we're not adding anything new to Forms anymore, but since the GTK backend is supported by the community anyway this shouldn't add to our load here.

Thanks for your contribution @JunielKatarn !