sushi-media-bin: Fix position update on fullscreen

Balló György requested to merge City-busz/sushi:fix-update-position into master

We have to add the tick callback to the overlay widget instead of the main widget, otherwise the position bar is not updated in fullscreen mode when the gtksink is used. This happens because we move the overlay between the main and fullscreen windows, and we did't get the events when the overlay is outside of the main widget.

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