Notify methods to invalidate steps

[MFlisar]

E.g. you may have a step that opens a system page where the user needs to set up something (e.g. to enable the accessibility service for the app) so when the user comes back and the app is resumed I need to recheck if the user did everything right and depending on this I need to update the current steps view.

Some extension to updateSteppers() like updateStep(index) would be nice to invalidate a single step only...

Additionally setting an error should probably only invalidate on step instead of all as well?