[SettingsCard] Change ActionIcon to utilize IconSource and IconSourceElement

[Avid29]

Currently, the ActionIcon is an instance of an IconElement. This is problematic, because the IconElement is a FrameworkElement, meaning it cannot be a shared resource, and therefore changing the icon requires a unique value for every instance. This means it cannot be set using styles.

The solution is to replace the IconElement with an IconSource, however this is a breaking change. As a result, a deprecation plan needs to be made

[niels9001]

I believe this was done so we'd have more flexibility in what to host ad an icon. Not 100% sure though as it's been a while.

What does NavView or SelectorBar use?

[Avid29]

Both the SelectorBarItem and NavigationViewItem use IconElement, but they have a default icon.

I'm not aware of any drawbacks to using IconSource, aside from introducing a breaking change. I can name a few reasons to change it though. I'm very much open to a discussion where we figure out all the pros and cons.

[UfBt12qJNZKcw]

To implement the proposed change from IconElement to IconSource for ActionIcon, we should consider how this will affect existing instances across the application, especially in styles where icons are currently reused. Additionally, it would be beneficial to define a clear deprecation path in the documentation to ease the transition for users relying on IconElement.

[niels9001]

If its a breaking change we should do this in a major release only.