Homogenize task creation behavior between the Gantt and Explorer tools

[ncouvert]

In the Gantt view, the "Create task" tool creates a task at the same level as the selected one.
In the Explorer view, creating a task from another task creates it as a child task.

The behavior should be consistent across both views.

It could be done by adding a new contextual menu action that creates a sibling task, or by changing it to create a child task.