[ptx] feature : image number on mouseover

[Rich]

it would be nice if preview window would display image number when mouse 
is moved at images (if several images overlap at that area, all 
corresponding image numbers would be shown).

sometimes i can see that a couple of images are aligned slightly 
incorrectly, but finding them at control points tab is hard (although it 
is possible to disable pictures selectively until correct ones are 
found, it takes a long time).

though this probably will be pretty hard to implement :)
-- 
  Rich