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>