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/defaults.c

@@ -537,6 +537,8 @@ WDefaultEntry optionList[] = {
 	    NULL, getPropList, setHotCornerActions, NULL, NULL},
 	{"WindowListAppIcons", "NO", NULL,
 		&wPreferences.window_list_app_icons, getBool, NULL, NULL, NULL},
+	{"MouseWheelFocus", "NO", NULL,
+		&wPreferences.mouse_wheel_focus, getBool, NULL, NULL, NULL},
 
 	/* style options */