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/spinctrl.h

@@ -35,6 +35,7 @@ public:
     wxSpinCtrlBase() {}
 
     // accessor functions that derived classes are expected to have
+    virtual wxString GetTextValue() const { return wxString(); }
     // T GetValue() const
     // T GetMin() const
     // T GetMax() const