Pygi-composite-templates

------------------
* Allow initialization of templates from GLib resources or from file

------------------
* Allow initializing multiple widgets

Breaking changes:
* No longer require user-data/object to be set in XML file, so if they're set
they will be passed to the signal handler as per usual
* GtkChild -> GtkTemplate.Child
* GtkCallback -> GtkTemplate.Callback

------------------
* Initial release, seems to work!