media-player: add seek buttons (v3)

Hopefully the final iteration 😉

Follow-up to previous approach !889 (closed), implementing what's proposed in !889 (comment 1279849).

Add seek icons from icon-library and change the media widget from:
  [prev] [play] [next]
to:
  [prev] [seek back] [play] [seek forward] [next]

Screenshot: