Add <theoplayer-seek-button>

[MattiasBuelens]

This buttons seeks the player forward or backward by a fixed offset.

Usage:

<theoplayer-seek-button seek-offset="-10"></theoplayer-seek-button> <!-- seek backward by 10 seconds -->
<theoplayer-seek-button seek-offset="10"></theoplayer-seek-button>  <!-- seek forward by 10 seconds  -->

On mobile, we show two of these buttons next to the centered play button in the default UI: