Adds new options for moving precedences and maintaining the positioning between dependent tasks

[stephenjmaher]

In an application for this gantt chart, I need the dependent tasks to be linked. As such, a change to one triggers a change in others.

Consider the following example where Task B "Write new content" depends on Task A "Redesign website".

If Task A is moved, or the ending time is changed, then that should result in Task B moving by the same amount.

Also, the precedent tasks are important. If Task B was moved or the start time was changed, the Task A should ,pve by the same amount.

To this end, this PR introduces two new parameters: move_precedences and maintain_dependency_position.

• If move_precedence is set to true, then the precedent tasks are moved in the same way that the dependent tasks were moved previously.
• The maintain_dependency_position parameter ensures that any change to tasks results in a move in the dependent tasks.
• If both are set to true, the changes in the dependent task start time will cause a movement in the preceding task.