Cleanup struct marshalling function names

Use a consistent naming scheme for struct marshalling cache related
functions. This removes prefixed underscores from function names
as well as redundant wording.

To ignore this commit with git blame use:
  git blame <this-commit-sha>^ -- gi/pygi-struct-marshal.c

https://bugzilla.gnome.org/show_bug.cgi?id=685218
Add marshalling coercion for Python classes and instances to GTypeClass

Automatically marshal Python GObject classes and instances to GTypeClass
structs (GObjectClass). This allows usage of the GTypeClass methods by
passing a Python GObject class or instance to the GTypeClass method.
This is needed to support usage of GTypeClass methods since we don't
manually bind GTypeClasses and they are not very well supported with
introspection.

https://bugzilla.gnome.org/show_bug.cgi?id=685218
Add GTypeClass methods as Python GObject class methods

Take all the methods from an objects type classs and add them
as class methods. For instance, GObject.ObjectClass.list_properties
is available as GObject.Object.list_properties().

Co-Authored-By: Simon Feltman <sfeltman@src.gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=685218
overrides: Make value argument to Container.child_get_property optional

Override Gtk.Container.child_get_property to optionally accept the "value"
argument. If "value" is not supplied, the override will locate the child
property value type and create the GValue. Additionally return the resulting
GValue converted to a native Python value.

https://bugzilla.gnome.org/show_bug.cgi?id=685076
overrides: Make value argument to Widget.style_get_property optional

Override Gtk.Widget.style_get_property to optionally accept the "value"
argument. If "value" is not supplied, the override will locate the child
property value type and create the GValue. Additionally return the resulting
GValue converted to a native Python value.

https://bugzilla.gnome.org/show_bug.cgi?id=685076
overrides: Add Gtk.Container.child_get/set overrides

Add overrides for child_get and child_set to Gtk.Container since these
are not introspectable methods.

https://bugzilla.gnome.org/show_bug.cgi?id=685076
tests: Don't use deprecated positional argument for Gio.Settings schema
Python 3.4 make check fixes

Bump GI required version to 1.39.0. This is needed to get rid of
expectedFailures which pass when built with 1.39.0 (unexpected successes
fail unittesting in Python 3.4).
Silence deprecation warning when using imp.reload.

https://bugzilla.gnome.org/show_bug.cgi?id=730411
release 3.13.2
configure.ac: post release version bump to 3.13.2