[PATCH] gitk: Handle MouseWheel events on Windows

[PATCH] gitk: Handle MouseWheel events on Windows

Windows, unlike X-Windows, sends mousewheel events by default to the
window that has keyboard focus and uses the MouseWheel event to do so.
The window to be scrolled must be able to take focus, but gitk's panels
are disabled so cannot take focus.  For all these reasons, a different
design is needed to use the mousewheel on Windows.  The approach here is
to bind the mousewheel events to the top level window and redirect them
based upon the current mouse position.

Signed-off-by: Mark Levedahl <mdl123@verizon.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>