While we're at it, also remove all the weird borders and use standard
icons. The timeline doesn't have a "baroque" theme, it's all flat for
now. If the need arises we can add some gradients here and there.

Change-Id: Ia9ce22d7f412c4999feca2284959be4d734267ac
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>