[Stunner] Improve sequences bendpoints usability

Users reported following issues/caveeats regarding bendpoints usage:

• Creating a bend-point feels hard
• Double-clicking a unselected sequence, sometimes adds a bend-point, sometimes just selects the sequence
• User expected to just click and drag
• Removing a bend-point does not remove it all the time and multiple retries are needed; perhaps the area is too small and the mouse event is not registered
• Dragging requires sequence being selected/highlighted