Seekbar support

[DanielOaks]

Could be good to have.

The library does support these functions: get_duration_seconds, get_position_seconds, and set_position_seconds, so we could probably use something along those lines to create a seekbar that automatically updates and is draggable (seeing how we call set_repeat_count as an example of calling those functions).

I might have a look at this, see how it goes.