Add "seek_seconds" function

Default "seek" function takes time in nanoseconds as argument which is not that useful, cause we will start playback from the nearest keyframe anyway. The new "seek_seconds" can take a double value for more precise seeking.

src/player.js

@@ -28,6 +28,11 @@ class GtkPlayer extends GstPlayer.Player
         this.widget.connect('destroy', this._onWidgetDestroy.bind(this));
     }
 
+    seek_seconds(position)
+    {
+        this.seek(position * 1000000000);
+    }
+
     _onStateChanged(player, state)
     {
         this.state = state;