Includes changes that need to be done at the time of the switch, including
for compatibility with the imported GTK3 supporting libgnomecanvas.

gantt-header, gantt-chart and usage-chart are ported to GtkScrollable
interface and their widget packing adjusted and simplified. The scrolling
sync between treeview and charts (and chart and its header) is simplified
by making use of GtkAdjustment object sharing.