Add disable_if_always_disabled() to Block objects.

This allows the theme to specify that a given effect only makes sense
if another effect is enabled; e.g. a crop that only makes sense if
immediately followed by a resize. This can cut down the number of
instantiations in some cases.

Also change so that 0 is no longer always the canonical choice;
if disabling a block is a possibility, that is. In situations with
things disabling each other transitively, this could reduce the
number of instantiations further.