Change scroll bar to resize like in GIMP

This fixes issue #33 (closed) and #51 (closed)

The main thing done was interpolating maximum scroll bar values, thus:

• At a scaling of X, the slider will be 1/X of the entire slide bar
• When going too far right or left, the slider resizes to a minimum of half it base size (so 1/2X when on a scaling of X)