Remove const-qualifications on ResponseSink arguments.

This allows ResponseSinks to mutate their internal state in reaction
to receiving a response, which isn't necessary in the actual program
but will make writing tests easier.
Add test cases for the emitter.
Move features to an init-list in player.cpp.

This is because our new emitter cleverly notes that there is
whitespace in MSG_FEATURES, and escapes it!  So we have to provide
it as a list of arguments now.
Announce file on load.

This *should* have happened previously--all the plumbing was there
for it--so I'm not sure why it wasn't there.