Add wxSpinCtrl::GetTextValue()

This allows to retrieve the current contents of the text entry part of
wxSpinCtrl.

For now provide a stub in the base class, will be made pure virtual
later.

Co-Authored-By: Vadim Zeitlin <vadim@wxwidgets.org>

include/wx/msw/spinctrl.h

@@ -62,6 +62,7 @@ public:
     void SetSelection(long from, long to);
 
     // wxSpinCtrlBase methods
+    virtual wxString GetTextValue() const;
     virtual int GetBase() const;
     virtual bool SetBase(int base);