Actions are added to the setups one by one now.

* Replaced setup_add_actions(), [bsetup|context]_add_actions() with
  context_add_action(), [bsetup|context]_add_action(), which only adds a
  single sensing result. This is simpler and also simplifies consistency
  checking (coming up).
* Removed situation and sensing results arguments from
  [b|k]context_init(). Usually one starts with the initial situation.
  Otherwise one can use context_add_action() to build the current
  situation, which may be pretty slow, though.
* Removed macros SF(...) and CONTEX_ADD_ACTIONS(...) because they are
  obsolete.
* Removed context_remove_undone_sf() because context_prev() now undoes
  the sensing already. The old context_remove_undone_sf() looked buggy
  and was untested anyway.