Stack tab
     
    This tab is used to control the stacking process :
    
      - The "x" an "y" numeric fields set the origin (in pixels unit) of the
	  crop rectangle (0,0 is at the bottom left corner).
 The rectangle position can also be (and usually is) changed by 
	  dragging it in the image frame.
 
- The "w" and "h" numeric fields set the width and height of the crop
	  rectangle.
 The rectangle size can also be (and usually is) changed by 
	  dragging its corners or border in the image frame, if the rectangle 
	  is not drawn, it can be created by dragging the mouse in the 
	  image.
 The crop rectangle is the region of the image which will be stacked 
	  in the final image. Parts of the image outside of the rectangle will 
	  be discarded.
- The "Double size" checkbox makes the stacked image be twice the size of 
      the crop rectangle. As the stacking is done with sub-pixel accuracy, 
      there is no pixellisation in the result.
      
- The "Monochrome" checkbox is used only when stacking flat field
      frames. It makes the stacked flat field monochrome ; which is very
      useful when your flat fields are color biased. If, for example, you use 
      images of the daytime sky (any similarities with my past tries would 
      be pure coincidence ). ).
- The "Stack" button starts to stack each image selected for 
	  processing (ie: with a checkmark) in the (almost) final image.
 Once started, it is impossible to select another tab and the "Stack"
	  button becomes a "Stop" button.
When working on dark frames or flat fields, the x, y, w, h fields
    are disabled and frame the entire image.
    When this tab is selected, the image frame displays the reference image. 
    The crop rectangle is drawn in red on this image.
    During the stacking process, each image, once added to the stack, is 
    highlighted in the list frame.
    Process tab ...