qtquick: Fix IndicatorWindow::posForIndicator()

Wasn't even implemented. Should make some tests pass.

src/private/DropIndicatorOverlayInterface_p.h

@@ -55,6 +55,8 @@ public:
 
     void hover(QPoint globalPos);
 
+    /// @brief returns the position of the specified drop location
+    /// The return is in global coordinates
     virtual QPoint posForIndicator(DropLocation) const = 0;
 
     static KDDockWidgets::Location multisplitterLocationFor(DropLocation);