Reorganize class TimePane

Take out the TimeScale item from the TimePane item. Make them both siblings and children of the TimePaneScene object. Position the TimePane item at distance equal to TIME_SCALE_HEIGHT from the top. With these changes, there will be then no need anymore for using this constant in the Timeline class.

Hopefully, the fix for this issue will allow a better management of mouse clicks in the graphics scene (see issue #123 (closed)).