Move logic from GTK callback to GTKOnTextChanged() virtual method

Check whether we should ignore the event and mark the control as being
dirty if necessary in a virtual method, which can be defined in
wxTextEntry and overridden by wxTextCtrl, instead of doing it in GTK
callback itself.

This will allow to reuse wxTextEntry callback for wxTextCtrl too in the
upcoming commits.

No real changes so far.

include/wx/gtk/textctrl.h

@@ -142,6 +142,8 @@ public:
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
+    void GTKOnTextChanged() wxOVERRIDE;
+
 protected:
     // overridden wxWindow virtual methods
     virtual wxSize DoGetBestSize() const wxOVERRIDE;