Add tutorial file for Glade 3.16-GTK3.10-Python 3

The two files are:

- A Python 3 file
- A Glade-XML file

The two files show a possible way of embedding a matplotlib-widget into a GTK-3.10 interface that was build using the Glade 3.16 interface designer.

The existing tutorial is for an older version of Glade and GTK and should probably be kept for reference (a lot of people still develop for those older setups). Another tutorial builds the whole interface using GTK3-code (which is the approach for less complex user interfaces).

I will try to make a more complex example next, which will use a few buttons that manipulate the matplotlib-plot. But this tutorial shows the bare minimum required to get this to work.
This text does not match the image 'mpl_with_glade_3' which looks like a standard mpl figure window to me.