This patch adds callbacks for use in C and Python
- callbacks which can be added in C (not related to python), each callback receives optional context, main, id and its own argument.

- python adds its own callback which scripters can access, this is a simple callback

def MyFunc(context, data): print("Callback:", data)
bpy.app.callbacks.load_post += [MyFunc]

The callbacks are cleared on loading new files as with drivers.

To keep the patch simple to review I've only added render and load callbacks.