Return empty rectangle from GetItemRect() if item is not visible

This was already the case if the item was not visible because its parent
was not expanded, but now make it also true for the items which are not
visible due to the current scrollbar position.

Add unit tests checking for this and also verifying that GetItemRect()
returns the coordinates in the physical window coordinates.

interface/wx/dataview.h

@@ -1448,8 +1448,9 @@ public:
     /**
         Returns item rectangle.
 
-        If item is not currently visible because its parent is collapsed,
-        return an empty rectangle.
+        If item is not currently visible, either because its parent is
+        collapsed or it is outside of the visible part of the control due to
+        the current vertical scrollbar position, return an empty rectangle.
 
         Coordinates of the rectangle are specified in wxDataViewCtrl client
         area coordinates.