Show child tasks in ui

[jespertheend]

In the properties window below a task configuration, the list of dependency tasks should be shown.
When running a task a spinner should be shown for every child task as it is running. This gives the user more feedback what process is currently running and where things are going wrong in case a task fails.

At the moment task dependencies are determined as a task is running. So we could update this ui while the task is running, but we might also add an extra method to tasks to fetch the list of dependency assets. That way we can show the list of child tasks even before the user presses the run task button.
In practice this is not so easy though, the script bundler task for instance collects all dependency files while the task is running, so getting the list of dependencies would be as expensive as running the task itself.