Add an authenticated property

[danielballan]

Let's add a property to the Context that lets us easily check whether the connection is authenticated (as opposed to anonymous). Basically, expose this expression as a property:

tiled/tiled/client/context.py

Line 177 in cb31d56

if (self.api_key is None) and (self.http_client.auth is None):

to enable:

>>> client.context.authenticated  # True or False