gitk: wheel scrolling multiplier preference

gitk provides scrolling of several windows, uses hard-coded values for
the amount of scrolling, and these values differ across platforms and
widgets. The nominal value used is either 1 text line per mouse /
touchpad / button event, or 5 lines. Furthermore, Tk does not scroll
text widgets by 1 line when told to, this usually gets 2-3 lines of
motion. The upper canvas objects holding the commit graph do scroll as
defined. But, clearly no value is universally preferred, so let's give
the user some control over this. Provide a single multiplier to be
applied for all scroll bindings, with a value of 3 to mean the default
nominal value of 3 line. This is selected both as a compromise between
the various defaults across platforms, and because it is the smallest
value honored by the two text widgets on the bottom of the screen.

Later commits will connect this variable for actual scrolling events.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>

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