added loop-around feature to timeline

Rhythm Narula requested to merge rhythm.narula26/pitivi:master into master

when we play all the clip and the red marker comes to the end of all the clips, now if play button is pressed again, it doesn't start from the beginning i.e doesn't loop around. In the corresponding changes, red marker start playing from the beginning when the play button is pressed after reaching the end of all the clips

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