set fix time values for skipping with left/right arrow key

[1uk4sh]

When watching media.ccc.de i can jump forward using the arrow keys (left/right).
I would expect to jump foreward some seconds, like with the actual button in the player.
Instead with some longer videos (~2h) one "right arrow key" skips 5-6 minutes.

It would be nice to adapt this value to some fixed time interval like 30sec, rather than to skip by percentage of the talk length.

CC: @binek who asked on twitter

[saerdnaer]

Related: #366