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>

interface/wx/spinctrl.h

@@ -133,6 +133,13 @@ public:
     */
     int GetMin() const;
 
+    /**
+        Returns the text in the text entry part of the control.
+
+        @since 3.1.6
+    */
+    wxString GetTextValue() const;
+
     /**
         Gets the value of the spin control.
     */
@@ -315,6 +322,13 @@ public:
     */
     double GetMin() const;
 
+    /**
+        Returns the text in the text entry part of the control.
+
+        @since 3.1.6
+    */
+    wxString GetTextValue() const;
+
     /**
         Gets the value of the spin control.
     */