[airstrike] Level changing and tags

[Ulf Ekström]

The tag dictionary should be cleaned when the level is unloaded, I
must have forgotten to do it. There is no function to do this at the
moment, though.
The objdict_clean() function removes all references to nonexisting
object, so we can use that function as Erik writes, if we do it after
the sprites have been killed. Alternatively we can delete the objdict
and make a new one. But I will add a function to just empty the
dictionary, it would be just

void objdict_empty(objdict_t *objdict)
{
objdict->size = 0;
}

Ulf