Clarification of the 'using derived widgets with glade' tutorial-patch

[Martin Nordholts <enselic gmail com>]

There is a cryptic formulation at the derived widgets with glade
tutorial at
http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/ch23s03.html

The concept of the BaseObjectType is a bit vague as it is now, here is a
patch that clarifies it a bit:
http://www.chromecode.com/temp/derived-widgets-with-glade-clarification.diff

- Martin Nordholts