Add Step Backwards key binding

[jidanni]

Mplayer provides

              . Step forward. Pressing once will pause movie, every
                   consecutive press will play one frame and then go
                   into pause mode again (any other key unpauses).

You forgot the step backwards key!!

One can step forward one frame.

But if one goes even one frame too far, the only way to go back one
frame, is to go back a whole 10 seconds (left arrow)!
MPlayer 1.3.0 (Debian)