"Drag" the page according to a mouse drag.
Drag scrolling moves the page according to the movement of the mouse.
You "grab" the character under the mouse and move it around.
If the mouse is clicked and released in the same place of time we
assume that the user didn't want to scroll but wanted to whatever
mouse-2 used to do, so we pass it through.
Drag scrolling is identical to the "hand" option in MacPaint, or the
middle button in Tk text widgets.
To test this function, evaluate:
(global-set-key [down-mouse-2] 'mouse-drag-drag) (fn START-EVENT)