Fix several bugs caused by using incorrect trace_height

For reasons discussed in issue #58, trace_height is one attribute of Geometry that is required by both GanttDiagram and Render.

Here, its value was not synced between GanttDiagram and Render. It caused the variables value text not to be displayed at the right height.

This is now fixed by syncing the values of trace_height in both GantDiagram and Render with the set_trace_height function.