Provide a drag-position control that reports two coordinates packed into
NewPosition. Construction prepends ButtonFrame, RelVerify InputMode and
ButtonBack background; later caller tags can override them. The class
creates or owns no Prop objects and installs no automatic scrolling
connection.
Setup first sets up Area and registers its own mouse-button handler.
SELECTDOWN inside the object enables mouse-move events, sets Selected
TRUE and records the mouse origin. Any later non-SELECTDOWN button event
removes mouse-move tracking and clears Selected. While selected, mouse
moves add displacement to the stored Horiz/Vert bases, pack the result
and issue a NewPosition setter for notifications.