gitk: update win32 scrolling for Tk 8.6 / TIP 171

gitk: update win32 scrolling for Tk 8.6 / TIP 171

gitk on win32 binds windows_mousewheel_redirector to all MouseWheel
events in the main window. This proc determines the widget under the
cursor, then determines what scroll command to give, possibly none, and
issues scroll commands to the widget. The top panes get only vertical
scroll events, as does the lower right Patch/Tree pane. All others get
both vertical and horizontal events. These are all hard coded at +/-
five lines.

We now have common MouseWheel event bindings that follow user
preferences for the scrolling amount, bind for only the five main
display widgets, and leave the other gui elements untouched. Let's use
this instead. With the scrolling preference set at 5, the users should
not notice much, if any, difference.

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