Support MediumIcon (24x24) size in RibbonGroupBox

[nishy2000]

If the RibbonGroupBox is disabled and collapsed state,
The icon will be displayed with the enabled appearance, not the disabled appearance.

How to reproduce this issue

1. Modify TestContent.xaml. Add IsEnabled="False" attribute to a RibbonGroupBox.
   e.g.

<Fluent:RibbonGroupBox Icon="{DynamicResource Fluent.Ribbon.Images.DefaultPlaceholder}"
                        IsEnabled="False"
                        KeyTip="ZXB"
                        x:Name="B"
                        Header="Spinners"
                        IsLauncherVisible="True"
                        LauncherClick="OnLauncherButtonClick">

2. Start Fulent.Ribbon.Showcase
3. Show the modified RibbonGroupBox item
4. Resize the window so that it is in the Collapsed state.
5. RibbonGroupBox Icon is displayed with the enabled appearance, not the disabled appearance.

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Environment

• Fluent.Ribbon 9.0.0 preview (latest develop branch [a657b61])
• Windows 10 Pro 20H2
• .NET Framework 4.6.2 / net5.0-windows