gtkmm branched: trunk is for gtkmm 2.13/14

[Murray Cumming <murrayc murrayc com>]

I have branched gtkmm, because I wanted to add some new methods.

svn trunk is for gtkmm 2.13/14 (though we don't yet know when a GTK+
2.13/2.14 will happen). I also moved the book (tutorial) and examples
into the gtkmm-documentation module.

svn branch gtkmm-2-12 is for the current stable gtkmm 2.12.
 
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