Make mouse wheel window focus configurable.

With commit e45a3bc07d a behavior change
was introduced that may disrupt the workflow of a user who may
intentionally want to give focus to a window without bringing it to the
front. Using the scroll wheel is less intrusive compared to the left,
right, and especially the middle (in the case of a terminal) mouse
buttons.

This commit introduces the ability to change the behavior by enabling or
disabling the ability to focus a window with mouse wheel in the expert
panel.

src/WindowMaker.h

@@ -378,6 +378,7 @@ extern struct WPreferences {
 	char auto_focus;                   /* focus window when it's mapped */
 	char *icon_back_file;              /* background image for icons */
 	char enforce_icon_margin;          /* auto-shrink icon images */
+	char mouse_wheel_focus;            /* allow focus window with mouse wheel */
 
 	WCoord *root_menu_pos;             /* initial position of the root menu*/
 	WCoord *app_menu_pos;