ENH: make apply_inverse realtime friendly + faster in loops

Currently, using `apply_inverse` in a loop is quite slow -- especially for realtime applications. Together with @LauriParkkonen, we figured out that if `prepare_inverse_operator` was not called every time one called `apply_inverse`, this could become much faster.
Please reorganize `verbose` to be last. Even if this constitutes an API change, it was a bug not to have it be last before.