Add a minimal implementation of Gtk.Template. See #52

This tries to add a minimal API which allows for basic template usage
with the possibility to maybe add more automation/subclassing/nesting
later on.

Compared to gi_composites.py this adds parameters to Child and Callback
to set the name, init_template() doesn't need to be called and is stricter
in what it supports to allow future improvements.

The _gtktemplate.py file should be resuable with older PyGObject versions
with the only difference that init_template() needs to be called.
13 jobs for gtk-templates in 8 minutes and 56 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build And Test
passed #23533
pypy2

00:01:48

passed #23534
pypy3

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passed #23528
python2.7

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passed #23524
win32
python2-mingw32

00:01:59

passed #23525
win32
python2-mingw64

00:02:01

passed #23529
python3.4

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passed #23530
python3.5

00:04:32

passed #23531
python3.6

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passed #23532
python3.7

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passed #23526
win32
python3-mingw32

00:01:54

passed #23527
win32
python3-mingw64

00:01:50

passed #23535
xenial-i386-py2

00:02:46

 
  Coverage
passed #23536
coverage

00:01:02