Use a ncurses timeout() of 1 second, which will block when waiting for input

We use 1 second so the position slider is refreshed correctly
We could use 500ms to not miss any change, now the slider can make a
2 seconds jump, see Nyquist-Shannon theorem

Add a variable to the context to exit the main loop as soon as the exit key
is pressed, so we don't have to wait one more second: vlc_object_alive()
doesn't return false immediately after calling libvlc_Quit()