Add notifications and sounds to schema

[rohansingh]

Add the ability to specific app notifications and sounds in the schema.

Here is an example:

def get_schema():
    return schema.Schema(
        version = "1",
        notifications = [
            schema.Notification(
                id = "notificationid",
                name = "Notification",
                desc = "A Notification",
                icon = "notification",
                sounds = [
                    schema.Sound(
                        title = "Ding!",
                        file = ding_mp3,
                    ),
                ],
            ),
        ],
    )

This change makes the Schema and SchemaJSON fields of Applet public, so that embedders can inspect the schema and access the sound files that are referenced by schema.Sound.

Also, the File type is moved to runtime/modules to avoid a circular dependency between runtime and schema.