Workfile Templates: add event system to Workfile Template Builder

[BigRoy]

Changelog Description

Add an EventSystem to the workfile template builder for placeholder plug-ins to register to.

From a placeholder plug-in you can now register to events:

def callback(event):
    print(event.data["depth"])
    print(event.data["placeholders_by_scene_id"])

class MyPlaceholderPlugin(...):
    def populate_placeholder(self, placeholder):
        self.register_on_finished_callback(placeholder, callback)
        self.register_on_depth_processed_callback(placeholder, callback)

Additional info

This is separated from #327 to make smaller PRs. Due to the event system being implemented here in isolation it may be hard to do a test run.

Once this is merged and #425 is merged I'll make follow-up PRs for Maya for plug-ins that actually use the event system.

Testing notes:

1. Workfile templates should still work.
2. Code should be reviewed.