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.