Always outter indicators if the center is obscured with something else

If there's some other floating window over our drop zone, then it's
very difficult to drop if there's no outter indicators.

Now we'll show the outter indicators if we suspect there's some other
window obscuring

src/private/DragController_p.h

@@ -50,6 +50,8 @@ public:
     void grabMouseFor(QWidgetOrQuick *);
     void releaseMouse(QWidgetOrQuick *);
 
+    FloatingWindow *windowBeingDragged() const;
+
 Q_SIGNALS:
     void mousePressed();
     void manhattanLengthMove();