gtkmm-documentation: Mark code snippets as not for translation

Submitted by Nicolás Satragno

Description

The gtkmm package includes the examples shown in the documentation. However, such examples are only in English. This is no minor issue since the code is really hard to understand when all the identifiers are in a language you don't understand. These examples are also included in the documentation itself. Although they could be translated there, they would be different to those included in the package. It would be a good idea to make a po file linked to the example code.