Show color dots in the exported Gantt chart

[dbarashev]

Scenario:

1. Create a task, change its color
2. Make the color column visible using the column manager dialog. This will show a colored dot next to the task name
3. Export the project to PDF or PNG

Expected: colored dots appear next to the task name in the Gantt chart part of PDF Export or in the exported image.
Actual: no colored dots

Related: #2309, #2660