Separate namespace for d3_event_dragSuppress.

Previously we were reusing the component’s namespace to suppress drag events;
however, this had the unintended effect of the drag suppress’s touchmove
listener being overwritten by the component’s touchmove listener. To prevent
this, the drag supression listeners now use a separate namespace.

This commit also switches to preventing the default behavior on touchmove rather
than touchstart, which still prevents panning and pinching while enabling click
events to still fire.