Fix determining the length of the text in wxTextEntry/wxTextCtrl (wxGTK)

Several functions of wxTextEntry and wxTextCtrl call to
gtk_entry_get_text_length() API to determine the length of the text in
GTKEntry. This API is available since GTK+ 2.14 so we have to implement
a fallback method for older GTK+ versions.
Dedicated function GTKGetEntryTextLength() is implemented in wxTextEntry
and exposed through its interface because it is also used in wxTextCtrl.

include/wx/gtk/textentry.h

@@ -80,6 +80,8 @@ protected:
     // Override the base class method to use GtkEntry IM context.
     virtual int GTKIMFilterKeypress(GdkEventKey* event) const;
 
+    static unsigned int GTKGetEntryTextLength(GtkEntry* entry);
+
 private:
     // implement this to return the associated GtkEntry or another widget
     // implementing GtkEditable