Let iAlways clear the spatial context

Right now, iAlways complains about the spatial context being non-empty. Same for iIntros "!#". IMHO it would be nice to have a way to say "please make the context empty if it isn't". I suggest for iAlways to do that. This is similar to the difference between iIntros "!>" and iNext -- the latter may affect the context, the former will not. Furthermore, this is backwards compatible because iAlways will now only succeed in strictly more cases.