Reduce code duplication in wxGTK wxDataViewRenderer classes

The code was more complicated than necessary, with the base class
providing both virtual GtkOnCellChanged() and GtkOnTextEdited() that
were both overridden to achieve the same thing, namely customizing how
the value entered by user is converted to wxVariant, in different
derived classes.

Make GtkOnTextEdited() non-virtual and remove GtkOnCellChanged()
completely and add a new simple GtkGetValueFromString() which is called
from GtkOnTextEdited() to do the conversion.

This removes the existing code duplication and will make it simpler to
modify this code in the future, without changing the behaviour.

include/wx/gtk/dvrenderer.h

@@ -47,9 +47,10 @@ public:
 
     // called when the cell value was edited by user with the new value
     //
-    // it validates the new value and notifies the model about the change by
-    // calling GtkOnCellChanged() if it was accepted
-    virtual void GtkOnTextEdited(const char *itempath, const wxString& value);
+    // it uses GtkGetValueFromString() to parse the new value, then validates
+    // it by calling Validate() and notifies the model about the change if it
+    // passes validation
+    void GtkOnTextEdited(const char *itempath, const wxString& value);
 
     GtkCellRenderer* GetGtkHandle() { return m_renderer; }
     void GtkInitHandlers();
@@ -78,14 +79,16 @@ protected:
 
     virtual bool IsHighlighted() const wxOVERRIDE;
 
-    virtual void GtkOnCellChanged(const wxVariant& value,
-                                  const wxDataViewItem& item,
-                                  unsigned col);
-
     // Apply our effective alignment (i.e. m_alignment if specified or the
     // associated column alignment by default) to the given renderer.
     void GtkApplyAlignment(GtkCellRenderer *renderer);
 
+    // This method is used to interpret the string entered by user and by
+    // default just uses it as is, but can be overridden for classes requiring
+    // special treatment.
+    virtual wxVariant GtkGetValueFromString(const wxString& str) const;
+
+
     GtkCellRenderer    *m_renderer;
     int                 m_alignment;