Moving examples and documentation into their own module

[Murray Cumming <murrayc murrayc com>]

I'm thinking of moving the gtkmm book and its examples (not the API
reference) into a gtkmm-documentation module, so we can have examples
that use libglademm and other higher-up stuff, without making gtkmm
depend on them.

We did this successfully for the maemomm-documentation module:
https://garage.maemo.org/plugins/scmsvn/viewcvs.php/trunk/maemomm-documentation/?root=maemomm

Any objections?
 
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