Add option for traversing a stack of giv files

[dov]

Giv currently lacks a nice interface for traversing a stack of giv files. Currently the only option is to put the files in lexical order in a directory, and then use the space and backspace keys to go back and forth between the files. If there are several thousand files, this is too slow. I suggest that a new keyword $giv be introduced that allow refering to multiple files that can be scrolled between by either up/down or by a scrollbar that will be shown.

Actually I think that the $image keyword is good enough, and I will change it to to not load the giv files, but just refer to them.

An interesting question is what is done if there is two different $image commands to a giv file and one to a giv file... In such a case up down can change the image and shift-up shift-down can change the overlay.