Redesign asset retrieval in the library.

Most assets now return a new class named `Asset`. This allows for the
assets to be consistently saved via a `save` method instead of special
casing for `Attachment`.

`AppInfo` is no longer a namedtuple it is a fully documented dataclass,
as well as having the state attached to it.

Fixes #1997

discord/widget.py

@@ -31,8 +31,6 @@ from .invite import Invite
 from .enums import Status, try_enum
 from collections import namedtuple
 
-VALID_ICON_FORMATS = {"jpeg", "jpg", "webp", "png"}
-
 class WidgetChannel(namedtuple('WidgetChannel', 'id name position')):
     """Represents a "partial" widget channel.