Indicate no-activity periods in the summary task bar

[dbarashev]

The idea is to make it more visible in the summary task bars if there are any gaps between the child tasks.

If there is at least one child task on a particular date, show that date in the summary task as it is now (solid black bar), but if there is no child task on that day, then show it differently.

Discussion on the support portal:
https://help.ganttproject.biz/t/wish-for-dotted-or-dashed-line-in-summary-task-representing-break-between-tasks/10224/6

[dbarashev]

Related: #1134