Nodal task UI arc editing

• Add a set of modes to the task panel with buttons to switch among them in the panel's title bar.
• Add a "select" mode to rubber-band select a set of tasks.
• Add a "connect" mode to insert arcs (dependencies and/or adaptors) into the task graph. This inserts a qtPreviewArc item into the task-panel's scene that illustrates a potential connection as users click on tasks.
• Implement an AddDependency operation that inserts a dependency arc.

Here is a short video of the system in action: task-arc-editing-2023-11-27_10.18.36