print debugging stuff

For each input command that is processed, print the key binding
it originated from.

Note that if a key combination is not mapped (like e-w), but a prefix
is (like e), the command bound to the prefix is run, but the debug
output will still show "e-w".

Special case the "ignore" command, so that mouse up events are
sent. (This will be needed for the on-screen-controller later.)

This is a mouse down event:
   Add cmd 'ignore' from 'MOUSE_BTN0' up_follows=1

And this is a mouse up event:
   Add cmd 'ignore' from 'MOUSE_BTN0' up_follows=0

If mouse buttons are actually sent as key presses (no
MP_KEY_STATE_DOWN / MP_KEY_STATE_UP flag, single event),
then you should see the key up version only.

For normal keys, the key down event should trigger the bound command,
as well as key repeat events.

For mouse keys, the key up event should do that. No key repeat events
should be triggered here.

Also test mouse double clicks.