Add wxCursor::GetHotSpot() and implement it for wxMSW and wxGTK.

Allow retrieving the coordinates of the cursor hot spot, at least for the
ports for which we know how to do it.

Closes #16539.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78134 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/cursor.h

@@ -39,6 +39,8 @@ public:
     wxCursor(int id) { InitFromStock((wxStockCursor)id); }
 #endif
 */
+
+    virtual wxPoint GetHotSpot() const { return wxDefaultPosition; }
 };
 
 #if defined(__WXMSW__)