GetCurrentSelection() was documented but not present in all ports (just MSW and GTK); moved it to the base classes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42727 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/combobox.h

@@ -59,6 +59,10 @@ public:
     virtual bool CanUndo() const = 0;
     virtual bool CanRedo() const = 0;
 
+    // may return value different from GetSelection() when the combobox
+    // dropdown is shown and the user selected, but not yet accepted, a value
+    // different from the old one in it
+    virtual int GetCurrentSelection() const { return GetSelection(); }
 
     // redeclare inherited SetSelection() overload here as well to avoid
     // virtual function hiding