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/gtk/spinctrl.h

@@ -35,6 +35,7 @@ public:
     // wxSpinCtrl(Double) methods call DoXXX functions of the same name
 
     // accessors
+    virtual wxString GetTextValue() const wxOVERRIDE;
     // T GetValue() const
     // T GetMin() const
     // T GetMax() const