Rewrite code for ordering and placing timelines in the time pane

For now, timelines are added sequentially to the time pane. There is some provision for getting them ordered when the config.yaml file is loaded. However, once they are created or added, their position becomes frozen. One possible way to fix this issue is the following:

1. When adding a timeline, always position it at (0, 0).
2. Add a new method (to the graphics object that is the parent of the timelines) for placing the timelines into the graphics scene according to their order property.

It will be better to wait until issue #128 (closed) is closed, before working on this issue.