Process key action immediately, kill the hotkey thread

This saves memory, diminishes latency and fixes an unlikely loss of
events if too many actions are sent. This may expose some re-entrancy
bugs, although I did some succesful smoke tests. Any path that emits a
key action or a key press needs to be careful. vout_thread_t.pf_manage
is the most obvious case. That should be safe as the vout thread holds
no locks when in pf_manage. Not sure about OpenGL though.

A small bug with SPU across vout change remains present as before.