Also connect on the gst_player::Player::connect_on_stream_end method.
The main reason for this is to be able to reset the slider bar to 0
upon a stream ends.
Now on_duration_changed requires a `Duration` type and
on_position_updated requires a `Position` type as oppose to
both accepting `ClockTime` as their argument.
Previously they depended on the player/pipeline to get the ClockTime
values, and only `PlayerWidget` had access to the `gst_player::Player`
object.
Now that it uses the gst_player methods instead of the raw pipeline
methods to get the ClockTime values it no longer needs access to the whole
PlayerWidget object.
It now uses gst_player::Player::connect_position_updated callback
to send, cross threads, the `position` value to the gtk main loop
which then updates the widget.
This now connect's directly to gst_player::Player::connect_duration_changed
method.
The method then sends a cross-thread msg to the Action channel in the main loop that
then updates the widget.
Should use the gst_player::Player callbacks instead but they require
the Send Trait which means we would need to use SendCell and that's
not something I am going to deal with right now.
This commit also removes the majority of the playback widget,
though most of it's code will make it to the PlayerWidget once
it starts to get wired to the gtreamer_plaer::Player.
