Sprites can now be edited even if offscreen via the sprite chooser.
However, the sprite chooser should be upgraded to show at least the
starting location as well as the image, so it is easier to decide which
to edit when choosing between two identical graphically sprites.
Added location information in Sprite selector when there is more than
one (both for editing offscreen sprites and editing with a click when
multiple sprites are in the region).

Changed logic for replacing sprites. If a sprite that is being edited is
at index n in the internal list, it is removed and the new sprite is
added at n using the insertion list iterator. This acts as a replace and
maintains order with other sprites. If more than 1 sprite have the same
icon and location, and even when they don't, when the sprite chooser
dialog pops up, having them always displayed in a consistent order is
important.