Fast path Python Property getter when accessing descriptor directly

Call the Python implemented fget() when a property is accessed directly
on a Python implemented GObject. This skips going through the GObject
machinery which ends up calling fget() and marshalling the results
through GValues.

https://bugzilla.gnome.org/show_bug.cgi?id=723872
Fast path Python Property getter when accessed through GObject interfaces

Break do_get_property() call into a re-usable function.
Call do_get_property() Python implementations instead of going through GObject
machinery for Python GObjects. This gives a performance boost for Python
GObject properties when accessed via. obj.get_property() and obj.props.

https://bugzilla.gnome.org/show_bug.cgi?id=723872