Add the ability to control the visibility of the expander button in the SettingsExpander control

[Lamparter]

As per files-community/Files#16349, it appears that the WCT settings expander control does not allow controlling the visibility of the expander button. It would be useful to make the visibility of the expander button customisable because:

• It would allow apps to disable the user from expanding and collapsing settings cards so only the app can automatically
• It would allow apps to include contents within an existing settings control (this is what Files wants), e.g.:
  

[JohnnyWestlake]

While nice, I wouldn't call is strictly necessary to add to the control. It is XAML, you can just remove the functionality from the template in your app.

[Lamparter]

That's right, but I think it would be better to add as a property for consistency

[niels9001]

Great feedback. Yeah, I've been looking for a way to have a SettingExpander look like a SettingsCard for the following usecase:

• SettingsExpander has a ToggleSwitch and a bunch of sub-settings related to that toggle as its Items
• When turned off, I want the SettingsExpander to look like a SettingsCard
• When turned on, it should look like a SettingsExpander and it's expandable