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.

WPrefs.app/Expert.c

@@ -130,6 +130,8 @@ static struct expert_option {
 	{ N_("Keep dock on primary head."),
 	  /* default: */ False, OPTION_WMAKER, "KeepDockOnPrimaryHead"},
 
+	{ N_("Allow windows to take focus using mouse wheel."),
+	  /* default: */ False, OPTION_WMAKER, "MouseWheelFocus"},
 };