Enhancement to the Pick plugin to have shape track the pick
Enhancement to the Pick plugin to have shape track the pick

This modifies the Pick plugin to have an additional option to center
on the pick.  When this setting is enabled, the pick shape is recentered
around the located center of the found object.  This is useful when images
are updating in the channel and you wish to keep "tracking" the picked
object.

Some additional changes in this PR:
- Box becomes the default shape, not Rectangle.  Boxes are drawn from the
  center of the object.  This should be more natural for ds9 users.
- Triangle has been removed as a shape for picks.  It's not a typical choice,
  and one can always use a Polygon to get a similar (or more useful) shape.

naojsoft/ginga,ejeschke/ginga,naojsoft/ginga,pllim/ginga,naojsoft/ginga,ejeschke/ginga,pllim/ginga,pllim/ginga,ejeschke/ginga