Fix scrolling in window decoration with hidden cursor.

Fix scrolling in window decoration with hidden cursor.

If the mouse cursor is hidden (by unclutter, for example), then scrolling
in the window decoration creates an event with a child
(i.e. event->child != XCB_NONE). This causes route_click() to be called
with dest=CLICK_INSIDE, which prevents scrolling through a stacked layout.

To fix this, check if a click is in the window decoration _before_
checking if the event has a child.