[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.