gitk: update aqua scrolling for TclTk 8.6 / TIP171

gitk: update aqua scrolling for TclTk 8.6 / TIP171

Tk provides MouseWheel events to aqua, similar to win32. But, these
events on aqua have a nominal motion value (%D) of 1, not 120 as on
win32. gitk on aqua provides specific bindings only for the top 3 panes,
giving a nominal scrolling amount of +/- 1 for all events. gitk includes
a hidden feature providing horizontal scrolling of the commit graph,
added in 5fdcbb1390 ("gitk: Fixes for Mac OS X TkAqua", 2009-03-23).
This horizontal scrolling is triggered by mouse events in any of the top
3 panes, and thus violates normal gui design where the object under the
mouse cursor scrolls.

Let's update this using the common bindings in 'proc bind_mousewheel',
allowing user preferences on motion scaling to apply to all windows.
The commit graph scrolling feature is removed by this, and will be added
back for all platforms in a later commit.

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