stbt-control: Wait between keypresses to avoid LIRC failure

If stbt-control sends keypresses to the LIRC daemon more frequently
than one keypress in 0.2 seconds, LIRC refuses the connection and the
script exits with "Resource temporarily unavailable".

To make it possible to send repeated keypresses by pressing and holding a
button on the keyboard, sleep 0.2 seconds between sending two remote
control keys and flush the buffer of the received keyboard keys.