Appending options to a disabled widget.RadioGroup shows them as enabled

[polarctos]

Describe the bug:

When options are added to an existing widget.RadioGroup that was previously disabled using (w *DisableableWidget) Disable(), the options are added in the styling for enabled elements.
Thus with a black text and icon instead of grey ones.
They also show the hover effect and are highlighted when clicked, but the option cannot actually be selected.

To Reproduce:

Steps to reproduce the behaviour: (see code below)

1. Create RadioGroup with some elements
2. Disable the RadioGroup
3. Append Options to the RadioGroup
4. See error, additional options are shown in black instead of grey

Screenshots:

Before, correctly shown as disabled:	Appended option is not shown as disabled:

Example code:

Full reproducer:

package main

import (
	"fyne.io/fyne/app"
	"fyne.io/fyne/container"
	"fyne.io/fyne/widget"
)

func main() {
	application := app.New()
	w := application.NewWindow("test")
	radioGroup := widget.NewRadioGroup([]string{"one", "two"}, func(string) {})
	radioGroup.SetSelected("one")
	radioGroup.Disable()
	button := widget.NewButton("Append three", func() {
		radioGroup.Append("three") // this option is then shown enabled
		radioGroup.Disable() // even disabling the group again does not help too
	})
	c := container.NewVBox(radioGroup, button)
	w.SetContent(c)
	w.ShowAndRun()
}

Device:

• OS: macOS 10.15.7 (19H114)
• Version: Catalina
• Go version: go version go1.16beta1 darwin/amd64
• Fyne version: fyne.io/fyne v1.4.2 h1:vh5P0ZIpczIAUu3uqh8YPjAPOy2XzgK7DcBD3CHj+3k=

[andydotxyz]

On develop for testing