Time bar interaction improvements

[generoso]

• [x] use two different color for the loaded part and the selected part (if different). For example you can use a grey with transparency for the selected part, and keep the black for the loaded part.
• [x] when clicking on a point of the bar, you should move the closest cursor in that point.
• [x] change the tooltip of the button "Synch Selection with [Current Tool]"